2) Meanwhile, sieve the flour and salt together, adding the egg, butter and yeast mixture. Mix well and knead until smooth. Cover and set aside in a warm place for 1 hour until doubled in size.
3) On a floured surface, knead the dough well then divide into 20 small pieces. Roll into flattened ball shapes, place a small amount of jam into each and carefully fold over, securing all of the edges to prevent the jam from escaping, and reshape into a ball. Set aside and repeat with the others.
4) Heat the oil in a deep saucepan until 150C (use a thermometer for this). Drop the balls in small batches for approximately 3 minutes, turning over halfway to ensure browning on both sides.
5) Prepare a sheet of baking paper near the oil and sprinkle with caster sugar and cinnamon. Lay out a piece of kitchen paper next to this. Once ready, use a slotted spoon to remove the balls and place on the kitchen paper to absorb any excess oil. Then transfer to the sugar paper and roll the bottoms in the sugar (avoid the tops). Leave to cool and repeat with your remaining balls.
6) Mix the icing sugar, lemon juice and zest together, then smooth out the consistency with drops of water until you have a fluid icing. Once cooled, dip the tops of the doughnuts into the icing and place on a wire rack to cool slightly before serving.