In the bowl of an electric mixer combine the butter, sugars, egg, and vanilla and mix until creamed and well combined, about 3 minutes (scrape down sides of bowl as necessary).
Add the chocolate chips, and beat on low speed until just combined, about 30 seconds. Add M&Ms and mix in by hand until combined.
Using a cookie scoop or your hands, scoop balls of dough using about 3 tablespoons of dough. Place about 2-inches apart on cookie sheets that have been lined with parchment paper sheet) and bake for about 10 minutes, don't overbake (they will still look slightly undercooked). Allow cookies to cool on baking sheet for about 5 minutes before transferring them to a wire rack.
Make sure you don't overpack your flour when you measure it, that's really important for this recipe. It's a fairly stiff/thick dough, so you don't want to over measure the flour and make the dough any stiffer. Make sure you scoop it and then level it off with a knife. Don't shake the measuring cup to level it off.
The great thing about these cookies, is you can change them up for any holiday by using holiday color M&M's or just use regular M&M's.
This is a fairly thick dough (meaning it isn't super wet), and doesn't really need chilling time, but I will sometimes chill the dough for 30 minutes. This just keeps the cookies even slightly thicker which I love. If you prefer a little bit flatter cookie, then make sure you gently flatten each dough ball with the palm of your hand before baking them.
You can place a few extra M&Ms on top before baking these cookies.