Preheat oven to 350 degrees. Grease 2 9x13 inch pans, then line with aluminum foil, leaving an overhang to pull the bars out after baking. Set aside.
In a large bowl, add the cake mix, pudding mix, canola oil, eggs, and milk and stir gently to combine. Batter will be thick. Stir in the M&M's®, then divide the batter between the two pans, pressing the batter down into the pan and corners until even. Bake for 20-25 minutes, or until golden brown on top, rotating pans halfway through baking if necessary.
Allow bars to cool for 15 minutes, then remove from pans and set on racks to cool completely. Once cool, spread the frosting across the top of one layer of bars, from edge to edge, then lay the second layer of bars on top. Cut into squares and enjoy!