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 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.