Preparation:

• Put the flour in a large mixing bowl and crumble over the fresh yeast.
• Add the warm milk, sugar, egg yolk, salt and melted butter and mix to a smooth dough using a wooden spoon.
• Cover the dough and let it rise at room temperature until it has doubled in size (1–2 hours).
• Divide the dough into equal balls about the size of a fist. On a floured work surface, roll the dough into tight balls then place on a floured baking tray. 
• Flatten the dough balls slightly, cover with a clean tea towel and leave to prove for 15 minutes.
• Meanwhile, heat the oil in a large, deep, heavy-based pan (do not fill the pan more than halfway) until it reaches 180C on a digital thermometer or until a breadcrumb tossed in the oil sizzles and turns brown in 30 seconds.
• Working in batches of 2–3, carefully fry the doughnuts in the hot oil.
• Fry for 2–3 minutes, then turn the doughnuts using a slotted spoon and fry for a further 2–3 minutes on the other side until golden brown all over and nicely puffed. 
• Remove the doughnuts using a slotted spoon and place on a baking tray lined with paper towels to drain.
• Put the icing sugar in a shallow bowl then carefully coat each doughnut in the sugar on both sides.
• Fill a piping bag with the jam. Use a skewer to pierce a hole in the side of each doughnut, then fill with the jam.
• Finish decorating your doughnuts however you like, using icing or whatever takes your fancy.