Step 1

Preheat oven to 350 degrees (F). Line a 9x13-inch baking pan with parchment paper. Spray parchment, and any exposed pan, with nonstick baking spray. Set aside.

Step 2

In a saucepan over medium heat, melt butter until browned. Remove from heat and cool for a few minutes, then scrape the butter into a large heat proof mixing bowl.

Step 3

Whisk in the brown sugar and mix until well combined. Stir in the vanilla. Then add in the eggs and egg yolk, beating until combined.

Step 4

Using a rubber spatula, fold in the flour and salt, mixing until just combined. Finally, fold in the M&Ms and chocolate. Scrape the batter into the prepared pan.

Step 5

Bake for 25 to 27  minutes, or until the edges are firm and golden and the center is just slightly wobbly. Cool in pan for at least 20 minutes before cutting. Sprinkle with flaky sea salt before serving, if desired.