Line a 9x9-inch baking dish with parchment paper or spray with nonstick spray. A 9x13-inch dish can be used for thinner squares.

In a large pot, melt the butter over low heat. Stir in the mini marshmallows once melted until creamy.

Turn off the heat and fold in the rice krispies until evenly mixed.

Fold in 1 cup of Lucky Charms marshmallows, mixing for a few minutes.

Press the mixture into the dish, smoothing the top with parchment paper. Top with the remaining Lucky Charms marshmallows.

Let the mixture set for about an hour before slicing into squares and serving.