Preheat oven to 350°F (177°C) and line a 9x13-inch baking dish with parchment paper.
In a large bowl mix together melted ½ cup unsalted butter, 2 cups brown sugar packed and 2 tablespoons vanilla extract. Then add in 2 large eggs and stir until batter is like a thick caramel.
In a separate bowl, whisk together 2 cups flour, ½ teaspoon baking soda and 1 teaspoon salt. Then pour flour into batter, roughly a ½ cup at a time, folding batter over itself to smooth any ribbons of flour into the batter. The batter will be very thick.
Add in 1 cup semi-sweet chocolate chips and 1 cup M&M's to batter and gently combine.
Pour into baking pan and spread evenly. Top with additional M&M's and chocolate chips if desired. Bake at 350ºF for 26 minutes.
Remove parchment paper from pan and set on a cooling rack to cool.
Use a knife or bench scraper to divide into 24 squares.
Notes
The cookie bars will be golden brown and firm, but soft and gooey on the inside. You can use a toothpick to see if the cookie bars are completely cooked. A little under done just makes these cookies extra soft and chewy.