peanut butter & chocolate cake w/ chocolate ganache & reeses pb cups

1 box (18.25 ounces) devil's food cake mix
3 eggs
1 cup buttermilk
1/2 cup vegetable oil
Good pinch of salt
2 cups mini Reese peanut butter cups, chopped, plus 1 1/2 cups more for garnish (about 2 - 12oz. bags)

12 ounces dark chocolate, chopped
1 cup heavy cream
3/4 cup creamy peanut butter
4 TB of confectioners sugar

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, vegetable oil and salt 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.
To make the PB ganache 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 confectioners sugar, whisking all till nice and smooth.
To frost the cake, you can first trim the tops of you cakes (with a serrated knife) first to make them flat, I didn't do this.
Put one cake layer on cooling rack and place on a baking sheet. Pour some ganache on top; spread evenly with a spatula. Then place remaining cake layer, and pour remaining frosting over the top, allowing it to spill over the sides. Smooth top and sides with spatula (see Note below).
Refrigerate 1 hour to set. Transfer cake to serving plate and garnish with the chopped Reese's PB Cups.

Note: I saved a bit of the ganache to put on the cake just before adding the chopped Reese's PB Cups to act as a glue, making sure everything adhered correctly, then chilled it again to let it set. So basically, frost the cake, then chill, then add a thin layer of ganache again, add the garnish, then chill again.