1 1/2cups(3 sticks) unsalted buttermelted & cooled until warm
2cupslightly packed light brown sugar
2eggs plus 2 egg yolks
1 1/2tablespoonspure vanilla extract
2cups12 ounces semisweet chocolate chips
Position the rack in the center position of the oven and preheat the oven to 350°F. Generously butter an 18x13x1-inch rimmed half sheet pan; set aside.
In a large bowl, whisk together the flour, baking soda, and salt; set aside.
In another large bowl, use an electric mixer to combine the melted butter, brown sugar, and granulated sugar. Blend in the eggs, egg yolks, and vanilla and beat until well combined. With the mixer on low speed, slowly and gradually add the flour mixture and mix until just combined. Stir in the chocolate chips and half of the M&Ms.
Spread the batter into the prepared baking pan, pressing down and smoothing the top into an even layer. Sprinkle with the remaining M&Ms and lightly press them into the surface. Bake for 18 to 23 minutes or until the edges are light golden brown and the center is set. Allow to cool in the baking pan before slicing and serving.
Just like brownies, these cookie bars will be softer in the center of the pan and crunchier around the edges, so bake accordingly for your preferred level of doneness.