Preheat oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.
In a large bowl, stir together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add eggs, milk, oil, and vanilla to the flour mixture and beat on medium speed until well combined.
Reduce speed to low and carefully add boiling water to the cake batter. Beat on high for about 1 minute.
Pour batter evenly into the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow cakes to cool in the pans for 10 minutes, then remove from pans and cool completely on a wire rack.
For the frosting, beat peanut butter and softened butter together until creamy. Add powdered sugar, vanilla, and heavy cream and beat until light and fluffy.
Assemble the cake by placing one layer on a serving plate and spreading with frosting. Repeat with the second and third layers.
Frost the top and sides of the cake. Decorate with chopped mini peanut butter cups and crushed peanuts. Drizzle with chocolate ganache if desired.
Prep Time: 20 minutes | Cooking Time: 35 minutes | Total Time: 55 minutes
