Line an 8x8 baking pan with parchment and set aside. Preheat your oven to 350°F.
In a light colored or stainless sauce pan, add your butter and place over medium heat. As the butter melts, it will first begin to bubble loudly, after this subsides it will begin to foam. Keep stirring as it foams to ensure the milk fats toast evenly and don't get stuck to the pan. When the butter becomes an amber color with a nutty fragrance, immediately transfer to a large bowl, making sure to scrape the toasted milk solids into the bowl as well.
Add your sugars and vegetable oil to your browned butter and mix.
Add your two whole eggs and one extra yolk to the sugar and butter mix and mix well until the eggs are thoroughly combined.
Add your salt and vanilla and stir together.
Add your cocoa powder and mix in.
Add in your flour and stir until just combined, no dry bits remaining.
Add in about 150g (~⅔ cup) of your m&ms and stir to evenly distribute them throughout the batter.
Pour your brownie batter into your prepped pan and smooth into an even layer.
Use the remaining m&m's and scatter them across the top of the brownie batter.
Place your pan on the center rack of your oven and bake for about 25 minutes. When brownies are done the top will be set and shiny but if touched it will still be quite soft.
Allow your brownies to cool in the pan for ideally at least two hours before cutting into them. They should be all the way cooled to room temperature to allow them to finish cooking and set. Store in an airtight container for 3-5 days, enjoy!
