Fresh yeast
Live baker's yeast pressed into a moist, crumbly block, usually sold as a 42 g cube.
In German-speaking countries fresh yeast sits in the chilled section of every supermarket, while in English-speaking countries it is best sought at a supermarket bakery counter, a local bakery or an Eastern European shop. It keeps about two weeks refrigerated and can be frozen in portions.
Substitute:
Dried yeast works well at about a third of the weight, roughly 7 g dried for 21 g fresh or two sachets for a full cube; the flavour barely changes, though the dough may be a little slower to start.