Hirschhornsalz (Baker’s ammonia, or ammonium carbonate, in English) is a traditional raising agent used throughout Germany and Scandinavia for Christmas cookies. Many moons ago Hirschhornsalz was made out of ground deer antlers, hooves and horns, but these days it’s chemically produced. Often used in butterless biscuit dough, Hirschhornsalz gives cookies a unique tight-knit, crunchy texture – best described as a cross between a sponge cake and cinder toffee. It is a key ingredient in such biscuits as Pfeffernüsse and Springerle. While Hirschhornsalz is readily available to buy in little sachets all over Germany come Advent time, it is harder to get hold of elsewhere and you may need to order it online. I have known people to substitute Hirschhornsalz with an equal mixture of baking powder and bicarbonate of soda (baking soda), but personally I think it’s worth seeking out and buying online should you not be able to find it in a local store.