In a large bowl cream together butter, white sugar, brown sugar, egg and vanilla with hand mixer. Now set aside the mixer and grab a rubber spatula or wooden spoon.
Add flour, salt and baking soda into the bowl and mix with spatula until a cookie dough forms.
Fold in M&M's and mix until they're equally distributed.
Preheat your oven to 350 degrees F. While the oven is preheating, place your cookie dough into the refrigerator.
Shape dough into large balls (about 2 tablespoons per cookie) on 2 baking sheets.
Bake for 13-15 minutes, or until lightly browned at the edges and bottom.