Special equipment: a kitchen scale; three 8-inch cake pans; a candy thermometer
For the cake: Preheat the oven to 325 degrees F.
Combine the flour, cocoa, baking powder, baking soda and salt in the bowl of a stand mixer fitted with the paddle attachment. Mix to combine. Mix in the sugar.
With the mixer on low speed, add the butter, a couple cubes at a time, and mix until completely combined. Slowly add the buttermilk and vanilla extract; mix until well combined.
Add the eggs and mix until smooth, 2 to 3 minutes, scraping down the sides bowl as needed.
Divide the batter among three 8-inch cake pans. Bake for 35 minutes, or until a toothpick comes out clean. Cool the cakes at room temperature for 1 hour, then freeze for at least another hour.
For the buttercream: Combine the sugar and water in a saucepan fitted with a candy thermometer. Heat over medium-high heat until the mixture reaches 240 degrees F (see Cook's Note).
Meanwhile, place the egg whites in a clean mixer bowl and beat until bubbles form. Add the sugar and beat on high speed until fluffy. Turn off the mixer.
When the sugar syrup reaches the target temperature, slowly pour it into the egg whites. Mix on high speed until thick and fluffy.
Gradually mix in the butter cubes until the mixture comes together and is fluffy and smooth.
Add peanut butter, vanilla and coconut extract and mix until well combined. (Makes 4 cups.)
To assemble: Trim the cake layers to level them; this will help you frost the cakes and ensure the layers are even. Spread the first cake layer with buttercream. Stack the second layer on top and spread with buttercream. Stack the third cake layer. Add the buttercream to a piping bag and pipe onto the top and sides of the cake. Smooth the frosting, using an offset or flat spatula. At this point, you can leave the frosting smooth or add some texture with the spatula.
Refrigerate the frosted cake for 30 minutes before slicing.
Pay close attention when boiling the sugar syrup: cooking it too long will cause lumps of sugar crystals in the buttercream.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.