Special equipment: a waffle maker for round waffles
Preheat a waffle maker that makes round waffles to medium heat. Prepare the cake mix according to the package directions.
Pour 1/2 cup batter into the center of the waffle maker, close the lid and cook until the cake is cooked through and springs back when touched, 2 to 2 1/2 minutes. Remove the cake from the waffle maker using a fork and set it aside on a wire rack to cool completely, about 10 minutes. Repeat with the remaining batter for 7 waffled cakes total.
Meanwhile, beat the butter and peanut butter together in a large bowl with an electric mixer on medium speed until light and fluffy. Slowly add the confectioners' sugar about 1/2 cup at a time, beating between additions, until incorporated. Pour in the heavy cream, vanilla and 1/2 teaspoon salt and beat until smooth. Finely chop 1 candy bar (about 1 cup) and fold in the pieces. Transfer the buttercream to a pastry bag or resealable plastic bag and snip the tip.
Put 1 waffled cake on a cake stand or plate. Pipe a layer of buttercream onto the cake, covering completely, and top with another waffled cake. Repeat with the remaining buttercream and waffled cakes.
Cut the remaining candy bars into ten 1/2-inch thick pieces (reserve any crumbs) and decorate the rim of the cake with them. Roughly chop any remaining candy bars and sprinkle them along with the reserved crumbs in the center of the cake, creating a mound. Chill for at least 2 hours to set completely. Bring to room temperature 1 hour before serving.
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