Preheat oven to 350 degrees F.
Line 13" x 9" baking pan with parchment paper. Set aside.
In a medium mixing bowl, whisk together the dry ingredients (flour, cocoa powder, baking soda, salt and cornstarch).
In a large mixing bowl, combine melted butter and both sugars and whisk well. Whisk for about 2 minutes to give the sugar a chance to start dissolving.
Add vanilla and egg and whisk until smooth.
Add dry ingredients and chocolate chips to butter mixture and stir with a spatula until a uniform dough forms.
Press dough into prepared pan. Sprinkle M&M's over dough and press gently.
Bake bars for 18 to 20 minutes or until the dough appears dry on top.
Cool in pan for 10 minutes, transfer onto cutting board. Cool completely, cut and serve.
Store in air tight container for up to 5 days.