Spray a 9’’ x 13’’ pan with non-stick spray or line with heavy duty aluminum foil greased with non-stick spray. If using foil let it drape over the sides of the pan for easy removal of bars.
In a large bowl measure out Lucky Charms. Take out 1/4 c. of the marshmallows from the cereal and set aside.
Remove two cups of mini marshmallows and set aside.
Place the butter slabs and remaining mini marshmallows in a large microwave safe container. Microwave for 2 minutes. Remove and stir until butter and marshmallows are melted and combined. Add vanilla and stir until combined.
Quickly mix cereal into the melted marshmallows and stir until it is combined. Add reserved mini marshmallows and stir until they are softened and partially melted. You do not want them to completely melt as the provides those delicious pockets of marshmallow goo.
Press mixture into prepared pan. You can butter your fingers to help press them into the pan with less mixture sticking with to them or use the butter wrapper to push the mixture into the pan. Damp fingers also work to help press the mixture into the pan without getting it stuck to your fingers.
Take the 1/4 c. reserved Lucky Charms marshmallows and press into the top of the bars. Then sprinkle with sprinkles if desired.
Let cool at room temperature for at least a hour.
If you used foil you can lift the treats onto cutting board and cut into pieces. Otherwise cut them in the pan.
Store in air tight container.