Preheat your oven to 350°F (175°C) and grease or line a 9x13-inch baking pan with parchment paper.
In a large mixing bowl, melt the butter in the microwave or on the stovetop until fully melted. Allow it to cool slightly.
Once the butter is cool, add the granulated sugar and whisk until well combined.
Add the eggs, one at a time, whisking well after each addition. Stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, and salt.
Gradually fold the dry ingredients into the wet mixture using a rubber spatula, being careful not to over-mix.
Gently fold in the M&Ms until evenly distributed throughout the brownie batter.
Pour the brownie batter into the prepared baking pan and spread it out evenly.
Sprinkle some additional M&Ms on top for decoration.
Bake in the preheated oven for 25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Once baked, remove from the oven and allow the brownies to cool in the pan for about 10 minutes. Then, transfer to a cooling rack to cool completely.
Cut into 12 equal pieces and serve.