Spray a 9x13" pan with cooking spray. Preheat oven to 350 degrees.
Melt butter in a large glass mixing bowl. Whisk in cocoa powder, sugar, eggs, and vanilla.
Add flour and salt and stir just until combined. Mix in the chocolate chips and 1 1/4 cups of the M&M's.
Spread batter into prepared pan. Sprinkle the remaining 1/4 cup of M&M's over the top of the batter.
Bake at 350° for 30-35 minutes, or until a toothpick comes out clean or with moist crumbs.
Notes
If you can't find mini M&M's, you can use any variety. Just keep in mind that the larger the M&M's, the more you need to add so that every bite has a candy in it!