Melt butter in a large pot. Add 1/2 bag of mini marshmallows and vanilla extract. Stir until completely melted. Take pot off of the stove. Add the Rice Krispies Cereal, brownie chunks, and 1 cup of additional mini marshmallow. Stir until incorporated. Spread into a buttered parchment lined 9 inch square pan. Cool completely and cut into squares.