Baking Powder (Best Option)
3 teaspoons baking powder per 1 teaspoon baking sodaBaking powder contains both an acid and a base, allowing it to produce carbon dioxide gas when moistened and heated, which leavens the cake similarly to baking soda. Since baking powder is less potent than baking soda, a higher quantity is needed to achieve the same rise.
To ensure success, reduce or eliminate any acidic ingredients in the recipe that were originally balanced against baking soda, as baking powder already contains acid. Monitor the batter consistency and baking time, as the rise may be slightly slower or less vigorous.
The final cake will be slightly lighter and less dense than with baking soda, with a mild difference in flavor due to the additional acid-base reaction within the baking powder. However, it remains the closest and most reliable substitute in chocolate cake recipes.