Preheat broiler with rack 4 in. from heat. Trim edges from brownies and save for snacks. Cut remaining brownies into 12 squares, each about 2 1/2 in., and set on a parchment paper-lined baking sheet. Place a piece of chocolate on each brownie with scored side down.
Arrange a variety of mini and regular marshmallows on each brownie to make faces: To make eyes, cut mini or regular marshmallows in half crosswise and press into chocolate, then place regular or mini M&M's into center of marshmallows. To make teeth, cut marshmallows into triangles and press into chocolate.
Place brownies under broiler and broil, watching carefully, until marshmallows are just browned and chocolate is a little melted, about 30 seconds, rotating pan if needed for even browning. Let cool a few minutes before serving.