The rule I learned from my Mom was to stll add some more of those ingredients when doubling, but to use less than a full double amount. IIRC, the ratio went something like: Double - 1.5x the normal amount of Baking Powder/Soda Triple - 2x the normal amount Quadruple - 3x the normal amount More than quadruple - not a good idea, make multiple batches instead. As I mentioned, I learned this from my mom, who learned it from her mom, who learned it from her mom, who learned it from a dinosaur. It is therefore entirely anecdotal and almost folkloric information. Any basis in actual science has been lost in the mists of time. It does work, though.
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I always do. A quick google showed other people have asked the same question as you, I couldn't think of a convincing reason to/not to double the baking powder other than you double everything else so it makes sense to. I copied in some other people's explanations below from wikianswers and yahooanswers. toshio: "Yes. Think of it this way: the reason for the baking soda is to make carbon dioxide, in the batter or dough or what have you and make bubbles, when the batter gets hot those bubbles expand thus rising what ever your making, but you need a certain amount, because if you have too little, you wont get that much rise, but too much and you will get too many bubbles and you will get something with possibly an unpleasing texture, or you could have an explosion in your oven. So yes since you double everything else then you will need to even that all out to get the cookies rise as well." emu: "If you are just going double, then you double everything, including the baking soda. If you are going to increase it a lot, like 25 times then science comes into play and you have to readjust the ingredient formula." cristina: "Yes, you double the whole recipe in it entirety. you need the baking soda to balance the ph and give the cookie lift. not a lot of lift just a little. you should re right the recipe in full before cooking and you will make fewer mistakes." Wow. That's a long answer. Enjoy!
Where did you hear that? It has to fall-down at some point... L