Prep your pan by lining a 9×13-inch baking dish with parchment paper or buttering it generously. Set aside.

In a large saucepan over low heat, melt 3 tablespoons of unsalted butter.

Add 4 cups of mini marshmallows and stir constantly until completely melted and smooth, about 4-5 minutes.

Remove from heat and stir in 1 teaspoon vanilla extract and optional green food coloring until evenly mixed.

Quickly pour in 6 cups of crispy rice cereal and stir gently to coat evenly.

Fold in 2 cups of Lucky Charms cereal carefully to keep marshmallows intact.

Press the mixture firmly and evenly into the prepared baking dish using a buttered spatula or wax paper.

Let cool at room temperature for at least 30 minutes until firm.

Lift the block using parchment overhang and cut into squares with a butter knife or bench scraper. Warm the knife under hot water and dry it before slicing for cleaner cuts.