Preheat oven to 350˚F. Mix together cake mix, butter and egg.
Press into a foil-lined, greased 9×13-inch pan.
Sprinkle with white chocolate chips and mint baking chips.
Top with one bag of dark chocolate mint M&Ms. (See note one.)
Pour can of sweetened condensed milk evenly over the top.
Bake at 350˚F for 25-30 minutes or until golden brown along the edges.
Allow bars to cool completely before cutting.
Notes
One: Mint M&MsIf you can't find mint M&M's, you can add 1-2 teaspoons of mint extract to the chocolate cake mix batter and use green M&M's instead.Two: StorageStore the cooled bars in an airtight container for 3-4 days.