Preheat oven to 350˚F and line a baking sheet.
In a medium bowl, whisk together the flour, baking soda, cream of tartar and salt. Set aside
In a large bowl with an electric mixer, beat the butter and sugar until light and fluffy.
Add egg and vanilla, beating until each has been incorporated.
Mix in marshmallows until evenly distributed.
Scoop cookies onto baking sheet about 2 tablespoons in size, a few inches apart.
Bake for 10 – 12 minutes, or until the tops have just set but haven’t started to turn brown.
Remove from oven and press additional marshmallows into the tops while cookies are still warm.
Allow the cookies to rest for a few minutes before transferring to a cooling rack. ENJOY!