Preheat the oven to 350 degrees. Use convection bake if you have it.
Place softened butter, brown sugar and granulated sugar in a medium-large mixing bowl and mix with a hand or stand mixer until light and fluffy.
Add vanilla extract, egg, cream of tartar, cinnamon and baking soda. Mix until well combined. Scrape down the bowl once complete.
Add the flour in two portions, mixing after each addition. Scrape down the bowl once complete.
In a separate bowl, combine the granulated sugar and cinnamon that will be used for topping the cookie dough.
Grab some cookie dough and form a 2-3-inch ball. Toss the ball in the sugar mixture and completely coat it. Then press it into the bottom of a well greased muffin tin (see photo above).
Repeat the step above until all of the muffin wells are filled (should make about 15 total cookie cups).
Bake the cookie cups for 15-20 minutes or until the edges just begin to brown.
Immediately after removing the cookies from the oven, use a tablespoon measuring spoon or a regular spoon to make indents in each cookie, forming your cookie cups.
Set the cookie cups aside to cool (do not yet remove them from the pan).