Bake brownies according to package directions, allow to cool completely.
With an electric mixer, blend together and beat butter for 4-5 minutes. Add cream cheese and whip for 1-2 minutes.
Add powdered sugar a little at a time, beating between additions until well incorporated.
Add in vanilla and peppermint extracts and continue beating.
Add in green food coloring a few drops at a time until desired color is achieved (I used about 8 drops).
Spread frosting in a thick layer on cooled brownies.
Sprinkle mini chocolate chips across the top of bars.
Cut into 24 square pieces and serve.