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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 , this is an undercooked cake. The key is to bake it just long enough so that the outside is firm while the centre remains liquid.

But exactly how long should that be? An equation can give us an idea. The teachers at how we can model the diffusion of heat using the equation , where L is the distance heat travels in centimetres, t is the time in seconds and D is the heat diffusion coefficient, a measure of how fast heat travels through a given substance.

For water, D is 0.0014 cm² per second. Since most foods are largely made of water, we can use this value to approximate how quickly heat will move through many types of food.

Let’s say that a chocolate cake is cooked for about 12 minutes (720 seconds) in a ramekin 8 cm wide. The equation tells us that the heat will travel about 2 cm through the batter in this time, giving us a nice thickness of solid cake on the outside and a decent molten core.

In their book , Pia Sörensen and David Weitz explain how to make an even more impressive dessert, similar to the one pictured, by covering a ball of ice cream in a thin coating and deep-frying it for just long enough to cook the outside while keeping the ice cream frozen.

Their recipe has a cooking time of about 15 seconds, during which our equation predicts that heat will travel 0.3 cm through water. But this recipe uses whipped egg whites to coat the ice cream. Like other foams, it is a great insulator and so should give the ice cream excellent, if brief, protection from the heat.

To give this recipe a try, scoop your ice cream into four 100-gram balls, place on a tray and put them back into the freezer for an hour. Whisk the egg whites until foamy. Then, separately, mix the crushed cornflakes and cinnamon. Roll the ice cream balls in the egg whites, then in the cornflakes.

In this time, the outside of the ice cream balls will already have started to warm up, so they need to go back into the freezer for a couple of hours to ensure they are completely frozen before frying. After this, heat your oil to 190°C in a large saucepan or deep fryer. Fry the balls, one or two at a time, for 15 to 20 seconds, then drain on paper towels before serving.

For other projects visit newscientist.com/maker.