HEAT oven to 350°F.
PREPARE cake batter and bake as directed on package for 13x9-inch cake; cool completely. Remove from pan; cut in half.
PLACE 1 cake half, top-side down, on plate; spread with about 1/3 of the frosting. Cover with remaining cake half, top-side up. Spread top and sides with remaining frosting. Press nuts into sides of cake. Keep refrigerated.