Spray a 9 inch square baking dish with non-stick spray or line with parchment paper
Add marshmallows and cubes of butter to a large microwave safe bowl. Heat for 1 minute in microwave, stir until melted and smooth. Heat for an additional 20 seconds if necessary.
12 oz bag marshmallows, 5 Tablespoons butter
Stir in vanilla extract until combined.
1/2 teaspoon vanilla extract
Add Lucky Charms cereal and mix until well coated.
6 cups Lucky Charms cereal
Spread mixture into prepared pan. Spray a spatula with non-stick spray if necessary and press into the pan.