Using a handheld whisk, beat together the egg yolks, sugar, flour and vanilla in a heatproof bowl.

Bring the milk, cinnamon and lemon zest to the boil in a medium saucepan.

Slowly pour the hot milk onto the egg yolk mixture and stir to combine. Return the milk and egg yolk mixture to the saucepan and cook gently over a medium to low heat until the custard is thick enough to coat the back of a wooden spoon.

Pour the custard into the lightly oiled cake pan – it should be about two centimeters/one inch deep. Leave to cool slightly, and then transfer to the fridge to cool completely for at least three hours. You can leave it uncovered.

Cut the chilled custard into four-centimeter/one-and-a-half-inch squares. Coat each piece first in a little flour, then in the beaten eggs, and lastly in the bread crumbs. Chill in the fridge for 30 minutes.

Heat the oil in a deep frying pan/skillet to 180 degrees Celsius/350 degrees Fahrenheit on a kitchen thermometer. Fry the coated custard squares until golden, turning once so they are golden all over. Remove with a slotted spoon and drain on paper towels.

Dust the custard bites with sugar and serve immediately.