Step by step
Add the water, milk, sugar, yeast, neutral oil, egg, flour and salt to the bowl of a stand mixer fitted with the Dough Hook. Mix on speed 1 for 5 minutes until a supple and sticky dough forms. Let rest in a warm place for 1 hour.
Punch the dough down, roll it out onto a floured surface and cut out rounds with a diameter of approximately 12 cm. Let rest for 10 minutes.
Put a dollop of custard on each round of dough. Make ball by bringing all sides of the dough together and pinching it closed.
Lower the donuts with a slotted spoon to a pot of frying oil of 180°C. Bake for 4 minutes, turning the donuts regurarly. Take out when all sides are golden brown.
Serve with a dusting of icing sugar.