Baking Soda and Cream of Tartar (Best Option)
1/4 teaspoon baking soda + 1/2 teaspoon cream of tartar per 1 teaspoon baking powderBaking powder is essentially baking soda combined with an acid, commonly cream of tartar. Using 1/4 teaspoon baking soda with 1/2 teaspoon cream of tartar mimics this balance, producing carbon dioxide gas during baking for proper leavening.
To ensure success, mix the two powders thoroughly before adding to the batter to avoid uneven rising. Avoid adding extra acidic ingredients to prevent over-acidification.
Cupcakes made with this substitute will have a rise and texture nearly identical to those made with baking powder, preserving the light crumb and tender mouthfeel expected.