Accidentally using baking soda instead of baking powder in a cake recipe can affect the final outcome of the cake because the two ingredients have different leavening properties. However, there are a few things you can try to salvage the cake:
Add an acid: Baking soda requires an acid to activate its leavening properties. If your recipe doesn't already include an acidic ingredient, such as buttermilk, lemon juice, or vinegar, you can try adding a tablespoon of lemon juice or vinegar to the batter. This will react with the baking soda and help create some lift in the cake.
Double-check the ratios: If you accidentally used too much baking soda, you may need to double-check the measurements and adjust the other ingredients accordingly. If possible, refer to a similar recipe that uses baking soda as the leavening agent and adjust the quantities accordingly.
Increase the mixing time: Baking soda requires more mixing time to activate its leavening properties. Give the batter a good whisking or beating to incorporate air into the mixture, which can help create some rise in the cake.
Reduce the baking temperature: Baking soda can cause cakes to brown more quickly, so you might want to reduce the baking temperature slightly to prevent over-browning or burning. Keep a close eye on the cake as it bakes and use a toothpick or cake tester to check for doneness.
Adjust your expectations: Depending on the extent of the substitution and adjustments, the texture and flavor of the cake may still be affected. It might not turn out exactly as intended, but it could still be enjoyable in a different way.
Remember, salvaging a cake after an ingredient substitution is not always guaranteed to yield the same results as the original recipe. It's always a good idea to double-check your ingredients and measurements before starting a recipe to avoid such issues.