Position two oven racks towards the center of the oven. Preheat the oven to 350°F. Spray 3, 8-inch round cake pans with nonstick baking spray and line the bottoms with parchment paper rounds.

In a medium mixing bowl, whisk together the buttermilk, eggs, oil, and vanilla until smooth.

In a large mixing bowl, sift the cocoa powder. Add the flour, sugar, baking soda, baking powder, and salt and whisk to combine.

Add the buttermilk mixture to the flour mixture and stir to combine. (The mixture will be thick. It’s ok if it’s not completely smooth at this point.) Add the hot water or coffee and whisk until the mixture is fully combined and smooth. Divide the batter evenly among the three prepared pans.

Place two cake pans on one rack and one on the other rack and bake for 20 minutes. Swap the pans between racks and continue baking for 10 to 15 minutes or until the cakes begin to pull away from the sides of the pans and the center springs back when gently pressed. Allow the cakes to cool in the pans for 15 minutes, then invert onto wire racks to completely.

In the bowl of a stand mixer fitter with the paddle attachment (a large bowl using a hand mixer), beat the butter and peanut butter together on medium-high speed for 2 to 3 minutes, until smooth and fluffy. Beat in the vanilla and salt until just combined.

With the mixer on low speed, gradually add half of the powdered sugar (300g). Scrape down the sides and bottom of the bowl, then add 4 tablespoons of milk.

Return the mixer to low speed and add the other half of the powdered sugar (300g). Increase the speed to medium and beat for about 2 minutes until fluffy. If the frosting feels too stiff or grainy, beat in the remaining 1 tablespoon of milk until it is smooth and spreadable.

Transfer 1 cup of frosting to a piping bag fitted with a large, closed, star tip. Set aside.

Place one cooled cake layer on a cake plate or cake stand. Top with ¾ to 1 cup of frosting and spread it into an even layer to the edges of the cake. Repeat with 2 more cake layers and frosting. Spread the top and sides of the cake with the remaining frosting to cover completely. Add any remaining frosting to the piping bag. Refrigerate the cake for 20 minutes so that the frosting firms up.

Meanwhile, place the chocolate in a small, heatproof bowl.

Warm the cream in a small pot over medium-low heat for 2 to 3 minutes, or microwave on high for 30 seconds until very warm and just steaming (do not let it boil).

Pour the warm cream over the chocolate and let it sit for 5 minutes. Stir until it is smooth and glossy. (If any pieces of chocolate do not melt, microwave at 50% power for 10 seconds, then stir once again. Repeat until all of the chocolate is melted.) Let it cool until just barely warm, about 15 minutes, stirring occasionally.

Pour the ganache over the top of the cake and spread it to cover the top and drip down the sides in a few places. Set aside to harden. (You can expedite this by putting it in the refrigerator for 10 minutes.)

Pipe the reserved buttercream around the top edge of the cake. Place 2 peanut butter cup halves between each dollop.

Refrigerate the cake for 30 minutes before slicing and serving.