When to use "baking powder" vs "baking soda"?

Hello! Thank you for your question, and what a good question it is, as there is a difference, and can result in a different outcome for your cookies. Baking Soda, also known as Bicarbonate of Soda, is 100% Sodium Bicarbonate. It is pure alkaline. Baking Powder on the other hand, is a mixture of Baking Soda, an acidic ingredient such as tartaric acid (cream of tartar), and a thickener such as cornstarch. You could even make your own Baking Powder by mixing these ingredients together in the right quantities. 

I hope that this helps, and please reach out if you have any further questions.