Spray with nonstick cooking spray a 9x13 baking dish- set aside.
In a medium pan over medium low heat melt butter.
Add in marshmallows. Stir until fully melted.
Once fully melted, remove from heat and stir in the Lucky Charms.
Press the Lucky Charms into prepared 9x13 baking dish. Use a spatula sprayed with non-stick cooking spray to firmly pack the lucky charms into the pan and then press srinkles on if using.
Let cool.
Let the lucky charms cool completely and then cut into 15-20 squares.