Line a 9 x 13-in pan with parchment paper and lightly grease with softened butter. Melt a stick of unsalted butter to bubbling, and add a 10 oz. bag of mini marshmallows. Remove from heat, add another 10 oz. bag of marshmallows along with a teaspoon of vanilla extract. When the marshmallows are melted, add 8 cups of Lucky Charms cereal, and stir until well-blended but still warm and gooey. Press evenly into baking pan, bake on center rack at 300 for about 20 mins. Cool at room temperature for an hour, and cut into squares.