In a medium size bowl, mix together the flour, cocoa powder, sugar and melted butter. Press this mixture into the bottom of an 8x8 square baking pan that has been sprayed with cooking spray. Bake at 325 degrees for 30 minutes.
After the shortbread has cooled, spread the chocolate buttercream overtop the shortbread. Sprinkle the M&M's on top.
Store in an airtight container.
We like a thick layer of chocolate buttercream on top of our bars. If you'd like these bars to be less sweet, you can make a 1/2 recipe of buttercream for a thinner frosting layer. Less calories that way too!The calories shown are based on the bars being cut into 12 pieces, with 1 serving being 1 frosted bar. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**