Place butter in large mixing bowl, melt in microwave.

Dump marshmallows into large bowl with the melted butter. Heart in microwave about 30 seconds, until warmed.

Stir butter and marshmallows to combine. While still hot stir in chocolate chips. Stir until combined.

Pour Cocoa Puffs into large mixing bowl of warm marshmallow mixture. Stir until evenly coated.

Press into lightly buttered cake pan. Allow to cool. Cut and serve.