For best results use a stand mixer. It's perfectly possible to do by hand but things are easier and fluffier with a machine. Combine the yeast, flour, sugar and salt, then stir in the milk. When combined, work in the butter a chunk at a time. Knead thoroughly or beat for a further 3 - 5 minutes.
Leave the dough to rise, covered, for an hour.
Get a large baking tray out to hold the dough balls. Dust it with a thin layer of flour. Portion the dough into even golf-ball sized pieces. For best results, use a very sharp, thin knife and weigh the dough balls. It should make around 24. When you have your balls ready cover loosely and leave to prove for a further hour.
Heat oil for frying to 175C. When ready, gently lower the dough balls into the hot oil and cook for about 2 minutes on each side, or until golden brown. Only cook about 3-4 at a time to avoid overcrowding the pan. Transfer to kitchen paper to drain while you finish the batch. Roll on a plate of sugar to coat.
For the filling, heat the jam gently in a saucepan and add a tablespoon of water to loosen it. Use a jam syringe to inject jam into the centre of the dough balls.
Put a pan of water on the hob on a gentle simmer and place a bowl on top. Break the chocolate into it and sprinkle salt into it. When melted, turn off the heat. Dip the doughnuts one by one into the chocolate to give them a little chocolate hat. Best eaten same day.