The easiest substitute for cream of tartar in cookies is baking powder. If the recipe calls for baking soda and cream of tartar, baking powder can be substituted for both. Other acids, such as lemon juice, can also be substituted.
Cream of tartar is an acid and one of the ingredients in baking powder. Baking powder is about two parts cream of tartar to one part baking soda. Therefore, when calculating, think about how much cream of tartar is needed. For instance, if the recipe calls for 1/2 a teaspoon of cream of tartar and 3/4 of a teaspoon of baking soda, the recipe will only need 3/4 of a teaspoon of baking powder plus 1/2 a teaspoon of baking soda. Don't try to substitute baking powder for cream of tartar if the recipe already calls for baking powder. According to The Kitchn, other acids, such as vinegar or lemon juice, can be used in place of cream of tartar, but they may change the texture of the cookie because a bigger quantity needs to be substituted.