Lightly spray a 9x13" baking pan with non stick cooking spray, then line with parchment paper.
Add the butter to a large pot set over low heat. When completely melted add the marshmallows stirring until completely melted.
1 stick butter, 1 bag mini marshmallows
Remove the pot from heat and add the cereal, stirring until evenly coated with the marshmallows.
1 11.5 oz box Lucky Charms cereal
Transfer the cereal mixture to the prepared pan and spread out in an even layer. Press it down gently, careful not to overpack it.
Let the lucky charms treats rest until completely cooled and mostly set.
Using the edges of the parchment paper remove the cereal slab from the pan and transfer to a cutting board.
Carefully cut into squares using a sharp knife, and serve.