Preheat the oven to 350°F (180°C). Butter two 8-inch cake pans, line the bottoms with parchment paper, then grease the sides and dust with cocoa powder.
In a large bowl, combine 1 3/4 cups all-purpose flour, 3/4 cup cocoa powder, 1 1/2 teaspoons baking soda, 1 teaspoon baking powder, 1 3/4 cups granulated sugar, and 1/4 teaspoon salt. Whisk to combine.
In another bowl, mix 1 cup buttermilk, 2 large eggs, 1/2 cup vegetable oil, and 1 teaspoon vanilla extract. Add to the dry ingredients and mix until well incorporated.
Stir in 1 cup hot coffee or water to the batter. The batter will be thin, but that’s expected.
Divide the batter evenly between the prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow cakes to cool completely in the pans.
Make the Frosting:
Beat 1 cup softened butter and 1 1/2 cups peanut butter until light and fluffy. Gradually add 2 1/2 cups powdered sugar and 1/3 cup heavy cream, continuing to beat until smooth and creamy.
Mix in 1 teaspoon vanilla extract. Transfer frosting to a piping bag or bowl and set aside.
Assemble the Cake:
If necessary, trim the tops of the cakes to level them. Place one layer of cake, flat side down, on a serving plate or cake stand.
Spread a layer of peanut butter frosting over the cake. Add the second cake layer, flat side up, and frost the top and sides of the entire cake.
Chill the cake for 2-3 hours to allow the frosting to set.
Make the Ganache:
Chop 3.5 oz of dark chocolate and place in a heatproof bowl. Heat 1/2 cup heavy cream in a small saucepan over medium heat until it begins to simmer.
Pour the hot cream over the chocolate and let sit for 1-2 minutes. Stir until the ganache is smooth and glossy.
Finish the Cake:
Pour the ganache over the top of the cake, allowing it to drip down the sides. Smooth the top with a spatula to make the finish even.
Pipe additional frosting decorations on top if desired, and garnish with extra chocolate shavings or crushed peanuts for a decorative touch.
