If desired, use 2 cups frozen (thawed) whipped topping instead of the heavy whipping cream.
It’s best to use a large box (20.5 ounces) of Lucky Charms cereal to make sure you have enough marshmallows.
Heat the oven to 375 degrees F. Spray a large cookie sheet with cooking or baking spray.
Stir the cookie mix, butter, and egg in a large bowl until dough forms. Stir in the cereal.
Press the dough onto the cookie sheet in a rainbow shape (about ¼-inch to ½-inch thick). Bake the dough for 15 minutes or until the edges are golden and center of dough is set. Cool completely.
In large bowl, beat the whipping cream with an electric mixer on high speed until soft peaks form. Carefully stir in the frosting and food color. Use an offset spatula to spread frosting over cooled crust. Top with cereal marshmallows in color-coded rows.