This Agate Cake features gorgeous, edible candy agate slices on top of a watercolor buttercream cake! You can use any color scheme you like for the agate slices and buttercream. Note that the candy will get soft and sticky when exposed to humid air or moisture (like in the buttercream), so it’s best to add the slices to the cake shortly before displaying and serving. They can be made days in advance and kept in an airtight container until ready to use.
Agate Cake Recipe
Prep Time: 2 HRS
Cook Time: 25 MINS
Total Time: 2 HRS 25 MINS
FOR THE AGATE CANDY SLICES:
- Assorted hard candies about 1-2 oz per large agate slice
- Non-stick heavy-duty foil
- Silver edible luster dust optional
- Vodka optional, if you’re using the silver luster dust
FOR THE CHOCOLATE CAKE:
- 6 oz butter at room temperature
- 4.75 oz granulated sugar (2/3 cup)
- 5 oz brown sugar (2/3 cup packed)
- 2 eggs at room temperature
- 2 tsp vanilla extract
- 8 fl oz buttermilk (1 cup) at room temperature
- 4 oz sour cream (1/2 cup) at room temperature
- 2 tbsp water or coffee
- 7.8 oz all-purpose flour (1¾ cups)
- 3 oz unsweetened cocoa powder (1 cup)
- 1½ tsp baking soda
- ½ tsp salt
FOR THE BUTTERCREAM:
- 6 fl oz pasteurized liquid egg whites (3/4 cup)
- 24 oz powdered sugar (6 cups)
- 1/2 tsp salt
- 24 oz unsalted butter at room temperature (6 sticks)
- 2 TBSP vanilla extract
- Gel food coloring I used Americolor Electric Purple, Violet, Sky Blue, Navy Blue, Electric Pink, and Super Red
TO MAKE THE AGATE CANDY SLICES:
- Take a square of nonstick heavy-duty foil, nonstick side up. Place a small bowl or cup on top, to serve as a guide, and fold up the edges to form a circular mold with at least a 1-inch lip around the center. Repeat, making different shapes and sizes, for as many candy agate slices as you want to make. Place the foil molds on a baking sheet, and preheat the oven to 350 F.
- Divide the candies by color. While they are still in their wrappers, use a meat mallet, rolling pin, or other large, heavy object to crush them into small pieces—the closer you can get to candy dust, the better! Undo the wrapper and pour the crushed candies into small bowls, divided by type of candy. The more varieties of color and opacity you can get, the better! After the candy is crushed, it will start to get sticky as it is exposed to the humidity in the air, so start to form the agate slices immediately.
- Place a few spoonfuls of one color into a foil mold, and use a small food-safe paintbrush to spread it out along the outer edges of the mold in an even layer. (You can use your fingers, but the candy tends to stick to skin, so a paintbrush is faster and less messy.) Make sure that the layer is not too thin—as the candy melts, it will thin out a bit, so make the layer at least ¼-inch tall if possible. Add a second, different color, and brush that one out to the edges next to the first. Continue to make concentric rings of candy pieces in the mold, alternating colors and opacities, until you’ve filled the whole mold with crushed candy. I found I liked it best if I worked in one color scheme (shades of blue, for instance, or warm-toned colors) for each agate slice, but you can experiment to see what works for you.
- Once all of the foil molds are filled, bake them in the oven at 350 F for 4-5 minutes. Check them after 4 minutes to see if all the candy pieces have melted. If some unmelted pieces remain, bake for another 30-60 seconds—just until it’s melted but the bubbling is minimal. Let the candy cool completely, then gently peel back the foil mold.
- If desired, mix some edible silver luster dust with a bit of vodka to make a colored paste, and paint a thin silver line around the edges of the agate slices. Store the slices in an airtight container until you’re ready to decorate the cake.
TO MAKE THE CHOCOLATE CAKE:
- Line four 8-inch cake pans with parchment, and spray them with nonstick cooking spray. Preheat the oven to 350 F.
- Add the butter and both sugars to the bowl of a large stand mixer fitted with a paddle attachment. Beat them together on medium-high speed, until light and fluffy, about 5 minutes. Add the eggs one at a time, beating well after each addition. Add the vanilla extract and mix it in.
- In a separate bowl, whisk together the buttermilk, sour cream, and water (or coffee, if using). In a different bowl, sift together the flour, cocoa powder, baking soda, and salt. With the mixer running on low, add a quarter of the sifted flour mixture. When the flour streaks have almost all disappeared, add a third of the liquid to the mixing bowl. When that’s incorporated, continue to add the drys and wets in an alternating pattern, ending with the dry ingredients.
- When the dry ingredients are nearly incorporated, stop the mixer. Scrape down the bottom and sides of the bowl with a rubber spatula, and finish mixing the cake by hand. Divide the batter between the prepared pans, giving each one about 11 ounces, and smooth it into an even layer. Bake the cakes for 22-25 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Cool the cakes completely before assembling.
TO MAKE THE BUTTERCREAM:
- Combine the whites, powdered sugar, and salt in the bowl of a large stand mixer fitted with a paddle attachment. Mix everything together on low speed, until the sugar is moistened and no dry patches remain. Turn off the mixer, scrape down the sides and bottom of the bowl with a spatula, then turn the mixer to medium speed. Beat on medium for 5 minutes.
- After 5 minutes, turn the mixer to medium-low and start adding the softened room temperature butter, 1-2 tablespoons at a time. Once all of the butter is incorporated, add the vanilla extract and mix it in. Stop the mixer and scrape down the bottom and sides once more. Turn the speed to medium and beat the buttercream for 10 minutes. At the end, you’ll have a frosting with a wonderfully light and creamy texture. It can be used right away, or stored in the refrigerator for up to two weeks, or the freezer for up to two months. If you have chilled the frosting, let it come to room temperature and then beat it for several minutes to smooth it out and restore its texture before using it.
- Decide what colors you want to use for your cake—I used blue, purple, and pink. Reserve 1 cup of white frosting, tint ½ cup of frosting black, then use gel food coloring to make a dark and light tone for each color you’re going to use (I made dark and light blue, dark and light purple, and dark and light pink.) Place each color in its own piping bag.
- Put a cake layer on a cake cardboard, and place it on a cake turntable. Pipe concentric rings of frosting, inspired by the dark and light bands of agate slices. My first frosting layer had a thin line of black frosting along the outside, then dark blue, then white, then light blue, then black, then a white center. You don’t have to do this exactly, just alternate colors in the same shade family to make concentric rings.
- Add a second cake layer on top, and do the same thing with a different color family: I used shades of red and pink for this layer. Add a third layer, and this time, use another set of colors (I used purple shades). Finally, top with your fourth cake layer. Spread a thin layer of frosting all along the top and sides of the cake, to lock in any crumbs, and refrigerate for 30 minutes until firm.
- Take the piping bags of colored frosting, and pipe dots and small lines of different colors of frosting all along the sides of the cake, until it’s nearly covered with a random assortment of different colors. Take a metal spatula or bench scraper and hold it against the side of the cake while slowly turning the cake on the turn-table. It will smooth out the frosting and cause the colors to blur together in a watercolor effect. Wipe off the excess frosting from the scraper, and repeat until the sides are smooth and you are happy with the design. If there are places that look too light or too dark, pipe a few dots of another color there and go back with the bench scraper to smooth it out.
- Do the same thing with the top of the cake, piping different colored dots and lines, then smooth it out with an offset spatula. Go along the edges and remove any excess buttercream from the edges of the cake. If you want to touch up the design, refrigerate the cake for 30-45 minutes, until the buttercream is hard, then go back in with a small spatula and blend in different colors onto the cake—it’s easier to get a nice blend if you’re working with a firm base layer of buttercream.
- Finally, press skewers or long toothpicks into the top of the cake so they stick out an inch or two, and use those to support the agate slices. Arrange the candy slices on top in an overlapping pattern. Serve at room temperature, and enjoy!
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