In a large bowl cream well together butter and sugar, add egg and beat until creamy, add vanilla.
In a medium bowl whisk together flour, baking soda and salt, then add M&Ms or chocolate chips.
To the large bowl add ⅓ at a time the flour mixture, mixing with a wooden spoon. Turn onto a lightly floured wooden board, knead gently until firm (with lightly floured hands), wrap in plastic wrap. Chill for 24 hours.
Pre-heat oven to 350°.
Remove from fridge, shape into balls and place on parchment paper lined cookie sheets, flatten with a fork (dip lightly in flour, so dough doesn't stick) and bake for approximately 20-25 minutes. Let cool completely. Enjoy!