Preheat oven to 350°.
In a microwave safe bowl, melt the butter and chocolate chips by microwaving in 30 second bursts and stirring between.
Pour the melted butter and chocolate into a large mixing bowl. Add the cocoa powder and both sugars. Mix until well combined.
Add the eggs and vegetable oil. Mix until combined, then continue mixing for 2-3 minutes. The batter should lighten and become glossy.
Add the flour, cornstarch, salt, and 1 cup of mini M&Ms. Mix until well incorporated.
Grease or line a 9x13 pan. Pour the batter into the pan and smooth with a spatula. Top with remaining 1/2 cup mini M&Ms.
Bake at 350° for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow brownies to fully cool, then cut and serve!