Preheat the oven to 350 degrees Fahrenheit.
Place the room temperature butter in the KitchenAid Stand Mixer. Then pour in the granulated and brown sugar. Cream on low for 1 minute until incorporated. Then, slowly add in the eggs and vanilla extract. Mix on medium for 1 minute.
Turn off the mixer. Add in the all-purpose flour, baking soda, and salt. Mix on low and then gradually increase speed to medium for 1 minute. Reduce the speed to low and add in the M&M's. Mix until just incorporated.
Spray a 9x13 glass pan with cooking spray. Using a silicone spatula, scrape the cookie dough contents into the glass pan. Smooth out until even. Add 1/2 cup M&M's to the top of the pan.
Bake for 30-35 minutes or until golden in color. Remove from the oven and let rest for 30 minutes before cutting. Store in an air tight container for up to 5 days.