In medium bowl, beat butter and peanut butter with electric mixer on high speed until creamy. Gradually add powdered sugar, beating on low speed until blended. Beat on high speed until light and fluffy. Spread frosting over cake. Refrigerate until frosting hardens, about 1 hour. With hot knife, cut cake into 6 rows by 4 rows. Cut each square diagonally in half. Top each cake bite with 3 peanuts.