Preheat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or foil, leaving overhang for easy removal. Lightly grease if using foil.
In a large bowl, whisk together the melted butter and sugar until smooth. Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, and salt. Add the dry ingredients to the wet mixture and mix until fully combined. Gently fold in the chocolate chips.
Spread the thick brownie batter evenly into the prepared pan and sprinkle the M&M’s over the top.
Bake for 25 to 30 minutes, until the edges begin to pull away from the pan and the top looks set.
Allow the brownies to cool completely before slicing into squares.