The first step is to butter or line a 9-inch square pan with parchment paper. Melt the butter with the marshmallows, set one cup of the marshmallows aside. Add the vanilla, stir constantly. Add the Lucky Charms® and stir until covered evenly. Now, stir in the one cup of mini marshmallows that were set aside. Scoop the mixture into the pan you are using. Butter your fingers to keep the mixture from sticking. I like to save several Lucky Charm® colored pieces for the top. This is when I put a few pieces of the Lucky Charms® on top. Let the mixture set up for about an hour. Cut as desired.