Line an 8x8 inch baking dish with parchment paper, or spray with nonstick cooking spray.
Pick out ¼ cup of the marshmallows from the Lucky Charms; set aside.
Melt the butter and salt in a large saucepan or pot over medium heat. Add the marshmallows and stir until the marshmallows have completely melted.
Remove from the heat and let sit for 1 minute. Since Lucky Charms have marshmallows, this will prevent the marshmallows in the cereal from melting. Meanwhile, spray a small plastic resealable bag with cooking spray on one side.
Pour in the Lucky Charms and stir to combine. Pour the mixture into the prepared baking dish. Place your hand inside the plastic bag and push the cereal down into the dish in an even layer. The cooking spray prevents the mixture from sticking to the bag.
Place the reserved Lucky Charms marshmallows on top and let cool for at least 30 minutes.
Lift the parchment paper out of the pan, place on a cutting board, and use a sharp knife to slice the Lucky Charms Treats into squares.
If not serving immediately, wrap the squares tightly in plastic wrap. Serve within 2 days.