Preheat oven to 350F. Grease three 8-inch cake pans (see note) and set aside.
In a large bowl, combine dry ingredients (flour, sugar, cocoa, peanut butter powder, baking soda, baking powder, and salt). Set aside.
In the bowl of a stand mixer, combine eggs, buttermilk (see note), oil, vanilla extract, and chocolate extract (see note). Mix to combine.
Add the dry ingredients gradually, mixing until just combined.
Add champagne and coffee (see note). Mix until smooth.
Divide the batter among the prepared pans (see note on weighing batter). If desired, affix wet cake strips to the pans (see note).
Bake 25-30 minutes, or until the tops of the cakes are set and no longer wiggle. Allow cakes to cool completely on a wire rack.
For Buttercream:
In the bowl of a stand mixer, combine egg whites and brown sugar. Place the bowl over a pan of simmering water on the stove, ensuring that the water doesn't touch the bowl.
Heat the mixture, stirring constantly, until sugar dissolves.
Once the sugar has dissolved, place the bowl on the mixer and use the whisk attachment to mix on medium-high speed until the mixture is stiff and the bowl is cool to the touch (about 10 minutes).
With the mixer running on medium-high speed, add the butter a few pieces at a time, allowing each addition to incorporate before adding the next. Continue whisking until smooth.
Add the peanut butter powder (sifted, if necessary) and vanilla extract. Whisk to combine. Frost the cake layers with the buttercream, reserving some buttercream to pipe on top, if desired.
For Ganache:
Combine chocolate and cream in a medium heatproof bowl. Microwave on 15-second intervals, stirring after each interval, until ganache is smooth.
While still warm and pourable, pour over the top of the cake, spreading the ganache with an offset spatula and allowing the ganache to drip down the sides, if desired.
After ganache has set, pipe reserved buttercream on top of ganache, if desired.Enjoy!
Notes
Note on peanut butter powder: This handy ingredient has all the peanut butter flavor without all the fat and heaviness of actual peanut butter. It's a baker's (and peanut butter lover's) dream. If you can't find it in your local grocery store, you can find it [here].Note on buttermilk: If you don't have buttermilk, you can make a quick substitution with milk (2% milkfat or greater) and vinegar or lemon juice. For this recipe, pour 1 ½ tablespoons lemon juice or vinegar into a liquid measure, then add enough milk to come up to the 1 ½ cup line. Let sit for 5 minutes, then it's ready for use!Note on chocolate extract: I highly recommend this (sometimes overlooked) extract! It lends amazing depth of flavor to chocolate baked goods! If you can't find it in your local grocery store, you can find some [here]. Even better, make yourself some [Homemade Chocolate Extract] for next time!Note on coffee: The cake won't taste like coffee, I swear! The coffee enhances the chocolate flavor, and I highly recommend it! If you would prefer not to, you can use hot water instead.Note on cake pans: This recipe is for a triple-layer 8" cake. If you don't have three pans, the batter will still be good at room temperature for about three hours if you need to bake them individually. See note on weighing the batter--it's much easier to divide the batter equally when you have a kitchen scale!Note on weighing batter: To ensure I have the same amount of batter in each pan, I use my kitchen scale (link [here]) and weigh the pans. It's hard to eyeball the amount when you're using three pans, especially if you don't have three and you're baking the cake layers individually.Note on cake strips: If you've never heard of cake strips, they're pretty handy. The wet strips affix to the outside of your pan, and help ensure your cake layers bake up with a flatter top, so you cut less cake away when you level the cakes. you can find them [here].