Chocolate Birthday Cake Butter Mochi

Ingredients

FOR THE BUTTER MOCHI:
12 tablespoons/170 grams salted butter, cut into chunks, plus more for greasing the pan
1 cup/173 grams semisweet chocolate chips
2 cups/403 grams granulated sugar
3 large eggs
2 teaspoons vanilla extract
1 (12-ounce/354-milliliter) can evaporated milk
1 (13.5-ounce/399-milliliter) can unsweetened coconut milk
3 cups/454 grams mochiko (sweet rice flour), like Blue Star brand
1 tablespoon baking powder
¼ cup/24 grams unsweetened cocoa powder
FOR THE FROSTING:
1 cup/250 grams creamy peanut butter
1 cup/201 grams Demerara or turbinado sugar
¼ cup/48 grams Pop Rocks candy, from 5 (.33-ounce/9.5-gram) packages, any flavor, for finishing (optional)
4 ounces/113 grams rainbow sprinkles, for finishing

Instructions

Make the butter mochi: Heat the oven to 350 degrees. Grease a 9-by-13-inch baking pan with butter.
In a microwave-safe large bowl, melt the 12 tablespoons butter and chocolate chips by microwaving on high in 30-second increments, stirring and repeating as needed, until just melted.
Add the granulated sugar to the melted chocolate and stir until combined. Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla and milks.
In a large bowl, stir together the mochiko, baking powder and cocoa powder until evenly distributed. Fold the liquid chocolate mixture into the dry ingredients, stirring until thoroughly mixed. When the batter is totally smooth, pour it into the prepared baking pan.
Bake until a toothpick inserted in the center comes out clean, about 1 hour. Let cool slightly.
As the cake cools, make the frosting: In a bowl with an electric mixer, whip together the peanut butter and Demerara sugar until the mixture has the texture of frosting.
While the cake is still slightly warm, spread the frosting evenly over the top. Let cool to room temperature. Just before serving, sprinkle with Pop Rocks, if using, and shower sprinkles over the top, then cut into squares and serve.