Preheat oven to 350℉. Grease three 8-inch round cake pans, line bottoms with rounds of parchment paper, grease parchment, then flour the insides of the pans, set aside.
In the bowl of an electric mixer, sift together the flour, sugar, cocoa powder, baking soda, baking powder and salt.
In a medium bowl, whisk together the eggs, buttermilk, coffee, oil and vanilla.
Add the wet ingredients to the dry ingredients and mix for 2 minutes on medium speed. Scrape the sides and bottom of the bowl and mix for an additional 20 seconds (batter will be very thin).
Divide batter evenly among prepared pans.
Bake for 20 minutes and rotate the pans in the oven. Continue to bake until a tester inserted into center of cakes comes out almost clean (with a few moist crumbs), about 12 more minutes.
Cool the cakes (in the pans) on wire racks for 20 minutes, then carefully turn them out onto cooling racks to cool completely.
Place one cake layer on a serving plate. Cover with 1 cup of the peanut butter icing and sprinkle with 10 of the chopped peanut butter cups. Top with a second cake layer and cover with another 1 cup of the icing and another 10 chopped peanut butter cups. Place the final cake layer on top face-down. Frost the cake with the remaining peanut butter icing, smoothing as best as possible.
Refrigerate the cake for at least 1 hour, until the icing is set.
Slowly pour the chocolate ganache over the top of the cake, letting it flow over the sides of the cake. Top with the remaining 10 chopped peanut butter cups.
Refrigerate the cake for at least 30 minutes, allowing the ganache to set up. Keep the cake refrigerated, removing it from the refrigerator 20 minutes prior to serving.
Peanut Butter Icing
Place confectioners’ sugar, peanut butter, butter, vanilla and salt in the bowl of an electric mixer fitted with a paddle attachment. Mix on medium-low speed until creamy, scraping down the bowl with a rubber spatula as needed.
Add the heavy cream and beat on high speed until the mixture is light and smooth.
While the cake chills, make the chocolate ganache. Place the chocolate in a 4-cup measuring cup; set aside.
Place the cream in a small saucepan over medium heat and warm until it just comes to a boil.
Pour the cream over the chopped chocolate and let sit for 2 minutes. Whisk until the ganache is smooth. Set aside to cool, whisking occasionally, until it has thickened slightly, yet still a pourable consistency.