Keyword baking powder, gluten free, homemade, vegan
The use of cups is mostly an indication of ingredients ratio. What is important is that the quantity of cream of tartar always needs to be twice as much as the quantity of baking soda. So you can also measure 100 grams cream of tartar and 50 grams of baking soda (and add a tablespoon of starch if desired). You could also use a mug or a drinking glass: full mug of cream of tartar, half mug of baking soda. What's important is that you use the same "container" to measure the two ingredients.