Preheat your oven to 350°F (175°C). Spray the bottom of an 8 x 8 inch baking pan with non-stick baking spray and line it with parchment paper, allowing the edges to hang over the sides by an inch or two for easy removal later.

Set up a double boiler by filling a medium saucepan with about 2 inches of water and placing it over medium-low heat. Place a heat-safe bowl on top, making sure it does not touch the water. Add the butter, chopped semi-sweet chocolate, light brown sugar, and granulated sugar to the bowl. Stir frequently until everything is melted and combined into a glossy but slightly grainy mixture. Remove from heat and allow it to cool slightly.

Add eggs one at a time into the melted chocolate mixture, whisking well after each addition. Continue whisking until the mixture lightens in color, becomes smooth, and slightly shiny. Stir in the vanilla extract.

Sift together the cocoa powder, salt, and flour. Gradually add these dry ingredients to the wet mixture and fold gently using a large spatula. Mix just until a few streaks of flour remain to avoid over mixing and maintain fudginess.

Pour the batter into the prepared pan and level the surface using an offset spatula. Sprinkle the M&M’s evenly over the top of the batter.

Place the pan in the preheated oven and bake for 45 to 50 minutes. The edges should be set, and a toothpick inserted about two inches from the side should come out with only a few moist crumbs attached. Avoid over baking to keep the brownies fudgy rather than dry and cakey.

Remove the pan from the oven and place it on a wire rack to cool completely. Once cooled, lift the brownies out using the parchment overhang and cut into 16 squares. Serve and enjoy!