Preheat oven to 350°F. Prepare 8x8 square pan with baking spray or a homemade pan release. You can also use parchment paper at the bottom for extra insurance and easy removal.
In a microwave-safe bowl, microwave the butter at 15-30 second intervals until completely melted. Set aside until ready to incorporate into your recipe.
Combine brownie mix, cocoa powder, and salt in a medium-size bowl and whisk together.
Now, add milk, egg, melted butter, vegetable oil, and vanilla extract.
Mix ingredients together on low with an electric hand mixer, or by hand with a silicone spatula. Make sure all ingredients are incorporating and not stuck on the bottom.
Add m&ms and chocolate chips. Gently fold in until evenly distributed.
Pour brownie batter into prepared square pan.
Top with extra m&ms and chocolate chips if you'd like.
Bake for 35-37 min. Use a toothpick or knife and insert into the center – if it comes out clean or with a few moist crumbs, it’s good to go.
Let brownies cool in pan for 30-45 minutes, and transfer to a wire cooling rack to finish cooling.