Bake brownies according to package directions on box. Cool completely. Cut into 16 squares.
Top each brownie piece with 1 large marshmallow.
Scoop frosting into a microwave-safe bowl. Microwave for 10 to 12 seconds. Stir. Top each marshmallow with enough frosting to cover it completely. Let cool for 2 minutes.
With a black edible marker draw on 2 circles for the eyes, and a circle for the mouth, on each marshmallow. OR if using decorating gel, pipe 2 dots for eyes, and a circle for the mouth, on each marshmallow. Serve!