Line a 9-inch square baking dish with parchment paper. Set aside.
In a large saucepan, melt butter over medium heat. Add in all but one cup of the mini marshmallows. Reduce heat to low and stir continuously until marshmallows are completely melted.
Remove from heat. Stir in vanilla extract. Add in the cereal and the remaining 1 cup of mini marshmallows, folding gently.
Pour into prepared baking dish. Use your fingers (dipped in water) to gently press the treats into the pan. Allow to set one hour.