Peanut Butter Fudge Cake
1 box (18.25 ounces) devil's food cake mix
1 cup buttermilk
1/2 cup vegetable oil
2 cups chopped chocolate-covered peanut butter cups (about 8 ounces), plus more for garnish
8 ounces dark chocolate, chopped
1 cup heavy cream
1/2 cup creamy peanut butter
Heat oven to 350 degrees F. Coat two 9-inch round cake pans with nonstick cooking spray. Line with waxed paper; coat paper.
In a large bowl, beat cake mix, eggs, buttermilk, and vegetable oil on low speed for 30 seconds. Increase speed to medium-high; beat for 2 minutes, scraping down side of the bowl after 1 minute. Fold in 2 cups of the chopped peanut butter cups.
Divide batter between prepared pans. Bake at 350 degrees F for about 34 minutes, or until a toothpick inserted in the center comes out clean. Let layers cool on a wire rack for 15 minutes. Turn out cakes onto wire racks and cool completely.
Place chopped dark chocolate in a medium-size bowl. Bring cream just to a boil and pour over chocolate. Whisk until chocolate melts. Add peanut butter and whisk until smooth.
Trim top of cake layers flat with a serrated knife. Put 1 cake layer on cooling rack and place on a baking sheet. Pour 1 cup frosting on top; spread evenly with a spatula. Top with remaining cake layer. Pour remaining frosting over the top, allowing it to spill over the sides. Smooth top and sides with spatula. Refrigerate 1 hour to set. Transfer cake to serving plate and garnish with chopped candy, if desired.