Use the extra butter to coat the bottom and sides of the sheet pan. Set aside.

In a large pot, melt the butter on low. Before the butter completely melts, add the marshmallows. Stir until the marshmallows are melting. Turn off the heat. Continue to stir until melted. Add the vanilla and green food coloring and stir. Add extra drops of color, if needed, until you reach the desired green color.

Add the cereal and stir until coated with the marshmallow mixture.

Place the mixture into the buttered pan. Place a sheet of waxed or parchment paper over the cereal mixture and press until evenly spread out in the pan. Use the cookie cutter to cut shamrocks. Press together any leftover mixture and repeat cutting shamrocks. Store in a container with waxed paper between layers to keep the shamrocks from sticking together.