Preheat oven to 375 degrees. Line an 8 inch baking pan with foil, and spray with non-stick spray.
In a large bowl combine melted butter, and cocoa powder. Mix until well combined. Add eggs, sugar, and vanilla mix well. Stir in the flour, cinnamon and salt until well combined. Fold in M&M's. Pour into prepared pan. Bake for 25 minutes, or until a tester comes out clean.