Preheat the oven to 350ºF. Line four 6-inch round cake pans with parchment paper and grease with butter or nonstick cooking spray; set aside.

In a medium bowl, stir the boiling water, espresso powder, and semi-sweet chocolate together until chocolate has dissolved and mixture is smooth. Set aside to cool.

In a large mixing bowl, whisk the flour, sugar cocoa powder, baking soda, baking powder, and salt together until combined.

In another bowl, whisk the eggs, buttermilk, vegetable oil, and vanilla together. Pour wet ingredients into the dry ingredients and stir until smooth. Add the cooled espresso-chocolate mixture, stirring until just combined.

Divide batter evenly among the four pans and bake in the oven for 30-35 minutes, or until a toothpick inserted in the center of the cake comes out clean. Remove from the oven and allow to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, peanut butter, salt, and vanilla together on high until creamy, about 2 minutes. Add the powdered sugar, a cup at a time, until fully incorporated. Mix on high for 2 minutes. Add the heavy cream and continue beating until light and fluffy.

Place a cake layer on a cake plate or pedestal. Spread a layer of frosting, then sprinkle a third of the chopped peanut butter cups evenly on top. Place another layer of cake on top, followed by another layer of frosting and chopped peanut butter cups. Repeat steps with the other cake layers, frosting, and chopped peanut butter cups. Chill cake in the freezer for 1 hour.

Once chilled and solid, frost the entire cake with a thin layer of frosting--this will act as the crumb coat. Chill cake in the freezer for another 30 minutes to set the frosting. Remove the cake from the freezer and frost it with a final layer of buttercream. Return the cake to the freezer while you make the ganache.

Make the ganache by placing the chocolate in a medium bowl. Pour the heavy cream into a glass measuring cup, and heat in the microwave until it starts to boil. Pour hot cream over the chocolate, cover with plastic wrap, and allow it to sit for a few minutes. Remove the plastic wrap and stir until smooth.

Using a small spoon, carefully ladle the ganache along the edges of the cake, allowing the ganache to drip down the sides.** Once the sides are done, pour a few tablespoons of the ganache into the center of the cake, and use an offset spatula to fill in the top of the cake.

Decorate the cake with peanut butter candies and goodies. Slice, serve, and enjoy!

Cake will keep covered in the refrigerator for up to 5 days.