Baking powder (also called "Alsatian yeast" or "chemical yeast" in France) is a chemical raising agent, mainly bicarbonate of soda (or sodium bicarbonate), which acts with heat to make cakes rise.
It is used in most cakes, like for example chestnut cake, added to the mixture, and put in the oven without delay.
It should not be confused with yeast, which is more usually used in breads and viennoiseries.
Note: French baking powder is not the same formula as British and is about twice the strength, so if you are using French baking powder in a British recipe, use half the quantity. If using British baking powder in a French recipe, double the quantity.
If you intend to buy: In France baking powder is sold in packets of 10 famous small pink sachets of 10 g. Always keep some in stock, in an airtight tin or box, as it keeps for several months.
[Translator's note: British cooks are more used to buying baking powder in tubs, and measuring by the teaspoonful. French recipes often specify it by the sachet.1 sachet = 2 level teaspoonsful, but because it it stronger, this is the equivalent of 4 teaspoonsful of British baking powder!]
If you want to use it: Baking powder is straightforward to use. The only thing it doesn't like is to be kept waiting, once mixed, before being cooked.