Line 8- or 9-inch round cake pan with foil; spray foil with cooking spray. In 4-quart nonstick Dutch oven, mix corn syrup and sugar. Cook over medium-low heat just until mixture boils. Remove from heat.
Stir in peanut butter until smooth. Stir in cereal and peanuts until coated. Press mixture firmly in pan. Cool completely, about 1 hour.
By lifting foil, remove cereal mixture from pan; remove foil. Place upside down on serving platter. Spread frosting over top. With toothpick, draw baseball pattern as shown. Outline pattern with decorating gel to resemble baseball.