any uses for leftover marshmallow frosting..made with egg whites cream of tartar sugar water vanilla ...can this be made into a cookie?? Than

I might suggest, for variety, dividing it and flavoring @ batch differently (ex: coffee, mint, almond, orange, all or one of these) and then using those icings as filling to sandwich 2 cookies together. Best to make the cookies fairly thin/flat instead of hunka, for ease in eating when filled and sandwiched.
Without studying the exact recipe, i don't know if it could be baked like a meringue kiss. but you could just bake off a sample and find out.