Guanciale

Cured pork jowl. Salted, peppered, sometimes rubbed with garlic, then air-dried for three months. Higher in fat than pancetta and considerably softer in texture; it renders into a glossy, almost translucent fond when warmed slowly.

The Roman quartet of cacio e pepe, gricia, amatriciana, and carbonara are all variations on the same five ingredients, and three of them depend on guanciale. There is no acceptable substitute, including pancetta.