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 a combination of baking soda (a base) and an acid (often cream of tartar). When combined with moisture and heat, they release carbon dioxide gas, which helps the bread pudding rise slightly and become tender.
For best results, mix the baking soda and cream of tartar thoroughly before adding to the custard to ensure even leavening. Avoid adding too much baking soda as it can cause a soapy taste.
This substitute closely mimics the original leavening effect, preserving the lightness and softness expected in bread pudding without impacting flavor.