WHEN cooking food, we need heat to diffuse from the outside to its centre. If we want food to be evenly cooked throughout, this can be a problem: by the time heat reaches the centre, the outside may be overcooked. But in some cases, we can use the slow diffusion of heat to our advantage, to create foods with a surprise in the middle.
One example is a molten chocolate cake, aka a chocolate fondant. Essentially, this is an undercooked cake. The key is to bake it just long enough so that the outside is firm while the centre …