1Heat oven to 350°F. Grease bottoms only of 2 (13x9-inch) pans with shortening or cooking spray. Make cake mix as directed on box, using water, oil and eggs. Spread batter evenly in pans.
2Bake 20 minutes or until toothpick inserted in center comes out clean. Cool completely in pans on cooling racks. Run knife around sides of pans to loosen. Place cooling rack upside down over each pan; turn racks and pans over. Remove pans.
3Place 1 cake on work surface or cutting board; spread grape jelly over cake to within 1/2 inch of edge. Top with remaining cake.
4In 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.