Using a small mixing bowl, whisk together the all-purpose flour and baking soda. Set dry ingredients aside.
Using a medium mixing bowl and a handheld electric mixer on medium-high speed, cream the butter for 1 to 1½ minutes.
Add the light brown sugar, granulated sugar, and vanilla extract. Continue mixing for 1 minute, until the butter mixture is light and fluffy.
Lower the mixer speed to low and add the egg, mix until well combined.
Keeping the mixer speed on low, slowly add the flour mixture to the wet ingredients. Mix just until the flour mixture is well incorporated.
Fold in the chocolate chips and M&M’s. Cover the bowl with plastic wrap and refrigerate the cookie dough for 1 hour.
Preheat the oven to 350°F. Line a cookie sheet with parchment paper, set it aside.
Remove the cookie dough from the refrigerator. Using a 1 tablespoon cookie scoop, scoop dough.
Roll the cookie dough into a ball. Space the dough balls 2 inches apart.
Place 5 to 7 M&M’s on top of each cookie dough balls.
Bake for 8 to 10 minutes, until the edges are golden brown. Let baked cookies cool on a wire rack.