Preheat your oven to 350°F (175°C). Grease and line a 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal.
In a mixing bowl, melt the unsalted butter. Let it cool slightly.
Stir the granulated sugar into the melted butter until well combined.
Add the eggs one at a time, whisking thoroughly after each addition. Then mix in the vanilla extract.
In a separate bowl, sift together the all-purpose flour, cocoa powder, baking powder, and salt.
Gradually fold the dry ingredients into the wet mixture using a rubber spatula until just combined. Be careful not to overmix.
Gently fold in 1 cup of M&Ms until evenly distributed in the batter.
Pour the brownie batter into the prepared baking pan, spreading it evenly.
Sprinkle additional M&Ms on top of the brownie batter for decoration.
Bake in the preheated oven for 25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Remove from the oven and let the brownies cool in the pan for about 10 minutes. Then, lift them out using the parchment overhang and place them on a wire rack to cool completely.
Once cooled, cut into squares and enjoy!