Coat an 8 x 8 inch square pan with cooking spray.

Melt butter on medium-low heat in a saucepan. Once melted, add mini marshmallows. Continuously stir until marshmallows are melted.

Remove from stove top and add in the vanilla. Then add the cereal and completely fold in until evenly mixed.

Transfer to baking pan and gently form into an even layer.

Sprinkle with extra marshmallows, if desired. Allow to cool before cutting.