I also recommend making this cake in advance so that it can have a chance to chill in the fridge for a while before you eat it. Eating a slice of leftover cake straight from the fridge is pure magic. So let's check out the recipe!
Yield: one 9" round layer cake
Source: Cake adapted slightly from Hershey's, Mousse adapted slightly from Serious Eats
1-3/4 cups of all-purpose flour
3/4 cup of cocoa powder (I used a blend of dutched and non-dutched cocoa, but regular cocoa works perfectly here)
2 cups of white sugar
1-1/2 tsp of baking powder
1-1/2 tsp of baking soda
1 tsp of salt
2 large eggs
1 cup of milk (I used whole milk)
1/2 cup of vegetable oil
2 tsp of vanilla extract
1 cup of boiling water
Using parchment paper, cut out two 9" circles to fit inside the bottom of the cake pans. Grease the two 9" pans, insert one parchment paper circle into each pan and flatten it on the bottom of the pan. Lightly grease the top of the parchment paper. Preheat your oven to 350℉.
In a large, heat-proof bowl, whisk together the flour, cocoa powder, sugar, baking soda, baking powder, and salt. Add the eggs, milk, oil, and vanilla. Beat well with a hand mixer on medium speed for 2 minutes. Then carefully stir in the boiling water. This will make your batter very thin; don't worry, it's supposed to do that!
Evenly divide the batter between the two greased and lined pans. Bake at 350℉ for 30-35 minutes, or until a toothpick inserted into the center comes out with just a few crumbs sticking to it and the cake is starting to pull away from the edge of the pan. Cool for 10 minutes or until cool enough to handle, then turn out the cakes onto wire racks to cool completely.
Peanut Butter Mousse:
4 oz of cream cheese, softened
1/3 cup of powdered sugar
1/2 cup of smooth peanut butter
1 tsp of vanilla extract
A small pinch of salt
1/2 cup of heavy cream
With an electric mixer, beat together cream cheese, sugar, peanut butter, vanilla, and salt until light and fluffy.
In a separate bowl, beat the heavy cream until stiff peaks form. Then fold the whipped cream into the peanut butter mixture. If made in advance, store the filling in the fridge until the cake layers are completely cooled and ready for assembly.
Dark Chocolate Ganache:
1/2 cup of dark chocolate chips (I used 60% cocoa)
1/2 cup of heavy cream
Place chocolate chips and heavy cream into a microwave-safe bowl. Microwave for 30 seconds. Stir. Microwave for another 30 seconds. Stir. Repeat as needed until most of the chips are melted (it will vary depending on your microwave). Stir until completely smooth. Cool slightly.
Be sure to remove parchment paper from the bottom of the cake layers. Place the first cake layer on a plate and cover with peanut butter mousse. Top with second cake layer. Pour chocolate ganache onto the top of the cake, and spread until smooth. Sprinkle chopped peanut butter cups or other garnish onto the ganache before it sets completely. Store any leftovers covered in the fridge.