Sieve the flour and rub in the fat until it resembles breadcrumbs.
Mix a small amount of the water with the yeast and the rest of the water with the salt, sugar and egg. Mix all of the solutions into the flour mixture.
Knead the dough until smooth. Allow to double in a warm place, covered with cling film.
Knock the dough back and divide into 8 portions. Shape and allow to rise again.
Deep fry for 8-10 minutes at 180°C, turning frequently until golden brown.
Remove from the oil and toss in caster sugar.
Once cooled, make a small hole in the side of the doughnut and pipe the rhubarb and custard into the centre.