Line a 9-inch square baking dish with parchment paper. Set aside.

In a large saucepan, melt butter over medium heat. Once melted, add in all but 1 cup of the marshmallows. Reduce heat to low and continue stirring until completely melted.

Remove from heat and stir in the vanilla. Add the cereal and the remaining 1 cup of marshmallows and gently stir until combined.