Put 250 g flour in a bowl and make a well in the flour. Crumble 10 g yeast, add 1 tablespoon sugar.
Warm 125 ml milk, add 1/3 of the yeast mix and pour into the centre of the flour. Mix to make a dough.
Cover the bowl and leave to stand in a warm place for about 10 minutes or until the dough has doubled in size. Wash 1 lemon and grate the peel. 50 g butter, cubed. Add 1 egg, butter, remaining milk, 45 g sugar and grated lemon peel to the dough. Knead the dough for the doughnuts with dough hooks of a mixer for 4 minutes and leave to stand in a warm place for 30 minutes until the dough has doubled in size. Place the dough on a floured work surface. Knead the dough again briefly and roll out the dough to about 1 cm thick. Using a doughnut cutter (about 5 cm Ø) cut out doughnuts and place on a floured baking tray. Cover the doughnuts for 5–10 minutes until they have doubled in size. Gently lower the doughnuts one at a time into a pan of hot oil (150°) and fry for 1–2 minutes until golden brown and then carefully turn over. Remove with a slotted spoon, drain on kitchen paper and allow to cool. Mix XUXU, icing sugar and 1 tablespoon lemon juice to make a smooth, thick glaze. Decorate the doughnuts with the glaze and sprinkles. Allow the glaze to dry and serve.