Lightly grease a 9x9-inch baking dish or line it with parchment paper. Set aside.

In a large pot, melt the butter over medium-low heat. Once fully melted, add the mini marshmallows and stir continuously until completely melted and smooth.

Remove the pot from heat and immediately pour in the Lucky Charms cereal. Gently fold everything together until the cereal is evenly coated with the marshmallow mixture.

Transfer the mixture into the prepared baking dish. Use a spatula (lightly greased) or wax paper to gently press it into an even layer—be careful not to press too hard or the bars can become dense.

Allow the bars to cool and set at room temperature for about 30–45 minutes.

Once set, cut into squares and serve.