Preheat oven to 350ºF. Line an 8x8-inch baking pan with parchment paper or aluminum foil, leaving some overhang. If using foil, spray well with cooking spray and set aside.
In a large bowl, whisk together the butter and sugar. Add in the eggs, making sure to mix well after each one. Mix in the vanilla until combined.
In a separate bowl, mix together the flour, cocoa powder, and salt.
Add the dry ingredients to the wet ingredients and whisk until well combined. Gently fold in the chocolate chips.
Pour the brownie mixture into the prepared baking pan (the batter will be very thick). Evenly sprinkle the M&M's over the top.
Bake for 25-30 minutes. You will know the brownies are finished when the edges start to slightly pull away and the top looks done.
Allow to cool completely and cut into squares.