Spray a 9 x 13 inch baking dish with non stick cooking spray.
Measure out the Lucky Charms and place in a big bowl.
Melt the butter in a large pot or saucepan over medium/low heat.
Add the marshmallows and stir until all are melted.
Gently fold in the cereal until it is completely coated with marshmallow.
Using a rubber spatula or your fingers, press gently into the prepared pan. Spray your fingertips with non stick cooking spray and it will be easier to spread around.
We like to reserve a few of the cereal marshmallows for the top of the bars.
Let set up for about 30 minutes. Slice and serve.
Notes
If you want to freeze them, place them in an airtight container with wax paper in between the layers. This prevents them from sticking together. They freeze well for up to 6 weeks.