In this recipe, substitute 2 teaspoons baking powder for the cream of tartar and baking soda. (gratefulprayerthankfulheart.com)
The easiest formula for making your own baking powder is: 1 teaspoon baking powder = 1/2 teaspoon baking soda + 1/4 teaspoon cream of tartar + 1/4 teaspoon cornstarch. (foodhero.org)
Is it maybe 1 teaspoon baking powder per cake? (bbcgoodfood.com)