Preheat your oven to 350 degrees F. Prepare three 8-inch round cake pans by greasing them and lining the bottoms with parchment paper. Grease the parchment and flour the insides of the pans, tapping out any excess flour. This will ensure your cakes come out smoothly.
In the bowl of an electric mixer, or using a large mixing bowl if you prefer a hand mixer, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt. This step is crucial for incorporating air into the dry ingredients, which helps the cake rise beautifully.
In a separate medium bowl, whisk together the eggs, buttermilk, coffee, oil, and vanilla. Make sure all your wet ingredients are at room temperature for better emulsification.
Add the wet mixture to the dry ingredients and mix on medium speed for 2 minutes. Scrape down the sides and bottom of the bowl with a spatula and mix for an additional 20 seconds. The batter will be thin, so don’t worry!
Evenly divide the batter among the prepared pans. Bake for about 20 minutes, and then rotate the pans in the oven to ensure even baking. Continue baking until a toothpick inserted into the center comes out almost clean, about 12 more minutes. You’re looking for a slight moist crumb on the toothpick.
Once baked, allow the cakes to cool in their pans on a wire rack for 20 minutes. Then, carefully turn them out onto the racks to cool completely. This step is important as it prevents the cakes from becoming soggy.
For the peanut butter frosting, place the powdered 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 as needed. Adding the heavy cream and beating on high speed until light and smooth is the key to a fluffy frosting!
If your cakes are domed on top, level them off with a knife. Place one layer of cake on a serving plate. Cover it with 1 cup of peanut butter frosting and sprinkle 10 of the chopped peanut butter cups over the frosting. Repeat with a second layer.
Place the final layer on top, face-down. Frost the entire cake with the remaining frosting, smoothing it out as best as you can. Refrigerate the cake for at least 1 hour to set the frosting.
Meanwhile, make the chocolate ganache. Place the chocolate in a 4-cup measuring cup and set aside. In a small saucepan, heat the heavy cream over medium heat until it just boils. Pour the hot cream over the chocolate and let it sit for 2 minutes before whisking until smooth. Allow it to cool slightly, whisking occasionally until thickened but still pourable.
To garnish, pour the ganache into the center of the cake, allowing it to flow over the sides. Top with the remaining chopped peanut butter cups. Chill for an additional 30 minutes to set the ganache. Before serving, remove from the refrigerator and let it sit for about 20 minutes to soften slightly.
