Preheat & Prep: Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking pan and line it with parchment paper, leaving an overhang for easy removal.
Melt Butter & Chocolate: In a heat-safe bowl over a saucepan of simmering water, melt the butter, semi-sweet chocolate, brown sugar, and granulated sugar. Stir until glossy and combined. Remove from heat and let cool slightly.
Whisk Eggs: Add the eggs one at a time to the cooled mixture, whisking well after each addition. Continue whisking until the batter is smooth and slightly lighter in color.
Incorporate Dry Ingredients: Stir in the cocoa powder, salt, and vanilla extract until fully combined. Gently fold in the flour using a spatula, stopping as soon as no streaks remain.
Add M&M’s: Fold in 2/3 cup of M&M’s, ensuring they’re evenly distributed.
Bake: Pour the batter into the prepared pan and smooth the top. Sprinkle extra M&M’s over the surface, pressing them lightly into the batter. Bake for 45-50 minutes, or until a toothpick inserted 2 inches from the edge comes out with a few moist crumbs.
Cool & Slice: Let the brownies cool completely in the pan on a wire rack. Use the parchment overhang to lift them out and slice into squares.