Sprinkle 1/2 cup cereal nuggets evenly in an 8-inch round cake pan. Combine remaining 1/4 cup cereal nuggets, chocolate ice cream, pudding mix, and peanut butter in a large bowl; stir until mixture is well blended. Fold in whipped topping. Spoon ice cream mixture into prepared pan, spreading gently with a spatula. Cover and freeze 2 hours or until firm. Let stand at room temperature 15 minutes before serving.