Mix the yeast with 2.5 ml of sugar and 30 ml of warm milk then rest for 15 minutes.
Sift the flour and salt into a bowl and stir in 15 ml of sugar.
Mix the yeast mixture, remaining milk, melted butter, and egg into the flour to form a dough.
Knead the dough and let it stand for 45 minutes until doubled in size.
Knead again for five minutes, divide into 12 balls, and rest for 30 minutes.
Heat the vegetable oil in a pan to 190°C.
Fry the dough balls in batches for 5 minutes until golden-brown and drain on kitchen paper.
Roll the hot doughnuts in the remaining sugar, slit the side, and fill with jam.