In a bowl: measure out 280 g of lukewarm milk, add 4 egg yolks and mix in 200 g of sourdough starter. Then add 100 g icing sugar, 2 packets of vanilla sugar, the grated zest of 1 lemon and sieve over 500 g flour.
Start kneading, then after a few minutes cover and leave to rest for half an hour.
After resting, add 80 g butter and 5 g salt.
Punch down the dough and put it in a greased baking dish for 1 hour.
Begin folding the dough every hour, the first folding is done with a petal fold, and then rest for 1 hour. These folds are needed to stretch the dough and produce air so that you get a fluffy, soft, airy crust after baking.
For the second folding, repeat the previous process and leave to rest again for 1 hour.
Fold the dough one last time and let it rest for 1 hour.
Roll out the dough to a thickness of about 1 cm and cut out to the desired shape.
Once they are cut out, cover them with a kitchen towel and leave them to rise in a warm place for about 2 hours.
Once the doughnuts are cooked, start heating the oil and once the oil is hot, fry the doughnuts for 3-4 minutes per side. As soon as you put the donuts in the oil, put a lid on the pan to keep the steam in. When golden brown, flip the doughnut over and remove the lid on the 2nd side.
When the doughnuts are golden brown, let them cool a little and then fill them with a meringue bag or serve with jam, icing sugar, whoever you like.