Preheat oven to 350 degrees F. Liberally spray a 13x9" baking pan with cooking spray. Set aside.

In a large saucepan, melt the butter over medium heat. Once melted, stir it around occasionally, watching it constantly, as it deepens and darkens in color. The mixture will bubble up, then start to look foamy, but keep stirring and checking it until it's nutty and fragrant and a deep amber color. Once achieved, immediately remove it from the heat and cool it for about 10 minutes.

Stir in the brown sugar until combined, then the eggs and vanilla. Stir in the espresso powder, followed by the baking soda and flour until a soft batter has formed. Stir in the M&M's candies.

Spread the batter into the prepared pan and sprinkle evenly with the sea salt. Bake for approx. 20-25 minutes or until a toothpick inserted near the center comes out clean. Cool completely before serving.