Preheat oven to 350°F. Grease and flour 2 8-inch round cake pans; set aside. Prepare cake mix according to package directions. Divide batter between prepared pans. Bake about 25 minutes or until tops spring back when lightly touched. Let cakes cool in pans on a wire rack for 10 minutes; remove from pans. Cool completely on wire racks.
Meanwhile, prepare frosting. In a large mixing bowl combine butter and peanut butter. Beat with an electric mixer on medium speed until smooth. Add milk and vanilla and beat on low just until combined. Slowly add powdered sugar, beating on low until sugar is just incorporated. Increase speed to medium-high and beat for 1 minute.
Use some of the frosting to fill and then crumb coat the cake. Chill the cake for 10 minutes to firm up the crumb coat. Generously frost the cake with the remaining frosting.
Make swirls in the frosting with the back of a spoon, using gentle pressure and lifting the spoon straight up at the end of each movement.
Cupping some of the peanuts in your hand, lightly press them around the sides of the cake. Once the sides are covered, use the peanuts that have fallen down onto the plate to press into any open spots.