Preheat oven to 350˚F and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking soda, cream of tartar and salt. Set aside.
In a large bowl with a mixer, beat the butter and sugar until light and fluffy.
Add egg and vanilla, beating until each has been incorporated.
Add in half the flour mixture and beat until combined.
Do the same with the rest of the flour mixture. Continue to mix until it forms a dough.
Stir in marshmallows.
Scoop cookies onto cookie sheet. I used a medium-sized ice cream scoop.
Bake for 12-14 minutes or until the begin to turn golden.
Remove from oven and press additional marshmallows into the tops while cookies are still warm.
Move to cooling rack for a few minutes to cool before serving.