You only need two ingredients to make this baking staple.
It happens to the best of us. You're at the grocery store buying ingredients for a baked good recipe, and you assume you have all the staples. But sometimes, you arrive home only to find you’re fresh out of baking powder. Though you might be tempted to use baking soda instead, the two are not interchangeable.
Baking soda is alkaline, meaning it reacts with an acid (for example, buttermilk in a cake recipe) to form carbon dioxide gas, which causes the cake to rise. Baking powder, on the other hand, has a dry acid mixed in. While it doesn't react in its dry state, the magic is activated as soon as a liquid is added.
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If you have cream of tartar and baking soda on hand, you're seconds away from homemade baking powder. Using a ratio of 2 parts cream of tartar to 1 part baking soda, stir the two together until combined, then use the mixture in place of baking powder. This works because cream of tartar is an acid (specifically, it's the byproduct of wine production).
Note that your mixture will mimic single-acting baking powder, as opposed to double-acting, which you're likely used to. What this means is that the baking powder will react as soon as liquid is present for one reaction. Double-acting baking powder reacts a second time once it’s in the oven and heat is present.
Because of this, you'll want to act quickly, since the baking powder will react as soon as you make your batter—and only then. In order to capitalize on all that captured gas, get your baked goods into the oven ASAP to guarantee they will rise.