For the Cake:
For the Peanut Butter Mousse:
For the Ganache:
Preheat the oven to 350 degrees Fahrenheit. Butter the bottom and sides of a 10" round cake pan, line the bottom with parchment paper and butter the parchment.
Melt the unsweetened chocolate in a double boiler over low heat, stirring constantly, until melted. Set aside. Sift together flour, cocoa powder, baking soda and salt in a medium bowl and set aside. In an electric mixer fitted with the paddle attachment, beat the butter and sugars together until smooth and creamy, about 3 minutes. Add the eggs one at a time, scraping the sides of the bowl with a rubber spatula as needed. And the vanilla extract. On low speed, slowly add the melted and slightly cooled chocolate. Add the flour mixture in three additions alternating with the buttermilk, beginning and ending with the flour mixture and only blending until just combined so as not to over-mix. Using a spatula, stir in half of the peanut butter cups (you can use large peanut butter cups too that have been chopped). Bake until the tops feel firm and a cake tester inserted into the center comes out clean, about 45-50 minutes. Cool on a wire rack until cool enough to touch and then invert the cake onto the wire rack to cool completely (use a knife around the edges to loosen the cake if necessary).
Prepare the ganache next. In a medium saucepan, heat the cream and butter over low heat until hot but not boiling. Add the chocolate chips, let stand for 30 seconds and then stir to fully melt the chocolate and blend the cream. Stir in the vanilla extract and set aside to cool and thicken.
To make the mousse, in the clean electric mixer fitted with the paddle attachment, blend together the cream cheese and peanut butter until smooth. Set aside. On low speed, slowly add the powdered sugar until incorporated. In another bowl, using an electric whisk, whip the heavy cream and vanilla until stiff peaks form. Add 1/3 of the whipped cream to the peanut butter mixture to lighten it and then gently fold in the remaining whipped cream using a rubber spatula, being careful not to over-mix and deflate the mousse.
To assemble the cake, invert the cooled cake (top side up again) onto a serving plate. Leaving a 1" edge all the way around, use a small, sharp knife to cup out the center of the cake to create a cavity but do not go all the way to the bottom. You are basically scooping out the middle bit of the cake. Spread half the mouse into the cavity then break up some pieces of the remaining cake and mound over the mousse piling into the center of the cake. Spread the remaining mouse all over the cake and refrigerate until firm, about 30 minutes. Then, using a spatula, spread the thickened ganache (it should have a yogurt-like thickness) over the entire cake and sides. Top with the remaining peanut butter cups.
Keeps covered in the fridge for up to 2 days.