Preheat oven to 350° and spray a brownie pan, or baking pan, with cooking spray just before filling with batter.
In a medium saucepan over low heat, add butter and chopped chocolate chunks and stir until melted.
Sprinkle cocoa powder into pan and stir until combined and smooth.
Remove from heat and let cool for 5 minutes.
Stir in sugar and vanilla until smooth.
Add eggs, one at a time, stir after each addition, then add flour.
Sprinkle 1 cup of mini MMs into batter and spoon into prepared pan.
Bake for 17-20 minutes or until toothpick inserted comes out mostly clean.
Remove from oven and let cool completely before frosting.
In a medium bowl, beat butter, powdered sugar, heavy cream and vanilla together until smooth.
Frost brownies as desired and top with remaining MMs.
Recipe mostly adapted from my Chewy Brownie Bites