Line a 9-inch square pan with parchment paper and liberally grease with softened butter. Set aside.

In a large saucepan over medium heat, brown the butter. As the butter melts, stir it with a rubber spatula to encourage even cooking. Once the butter has fully melted, it will alternate between rapid large bubbles and gentle small foamy bubbles. The butter will be ready once it turns a dark golden color and the fat solids have darkened and settled at the bottom of the pan.

Once the butter has browned, turn the heat down to low and add the 12 oz bag of marshmallows and vanilla. Stir frequently until fully melted, then remove from heat.

Immediately fold in the lucky charms and rice krispies cereals until nearly fully coated in the melted marshmallow. Lastly, mix in the reserved lucky charms marshmallows and additional mini marshmallows.

Moving quickly, pour the rice krispie treat mixture into the prepared pan. Grease the bottom of a small measuring cup or spoon with butter and use to pack the mixture into an even layer. Top with additional lucky charms marshmallows, if desired.

Cover the rice krispie treats and allow to set at room temperature for one hour or in the fridge for 20-30 minutes.

Use a sharp knife greased with butter to cut the rice krispie treats into 16 squares, and enjoy!