Preheat oven to 350 degrees F.
In the bowl of a standing mixer, or a large mixing bowl, combine the butter, sugars, eggs, and vanilla.
Beat on medium high for 1 minute. (An electric mixer works best.)
In a separate bowl, add the baking powder, baking soda, cream of tartar, salt, and flour.
Whisk gently to combine.
Add the flour mixture to the sugar/egg mixture in 3 increments, mixing well and scraping the bowl between additions.
Put cookie dough into prepared pan. (See recipe notes.)
Before baking, top cookie dough with M & M's if desired.
Bake for 25 minutes.
May be served warm or at room temperature.