Spray a 9 x 13 baking dish with cooking spray and line with parchment paper and set aside.

Start by melting a stick of unsalted butter in a large pot over low heat. Add a quarter teaspoon of salt to balance out the sweetness.

Add mini marshmallows to the pot and stir constantly until it melts. Don't worry if there are small lumps of marshmallows remaining.

Once the marshmallows have melted, remove the pot from the heat and add 10 cups of Lucky Charms cereal. Fold the cereal together with a wooden spoon until it's evenly coated with the melted marshmallow mixture. Be careful not to crush the cereal while stirring.

Gently spread the mixture into a lightly greased 9x13 baking pan. Use a rubber spatula to press down. Don't press it too hard, or you'll end up with hard treats

Immediately top with extra Lucky Charms marshmallows and rainbow sprinkles.

Remove from pan and cut into squares.