Sift the flour and salt together and add yeast.
Rub in butter until mixture resembles fine breadcrumbs.
Combine egg and milk.
Add to dry ingredients, mixing to a soft dough.
Knead for about 10 minutes until dough is smooth and elastic.
Place dough in an oiled bowl, cover and allow to rest for about 30 minutes.
Knock down and divide into 12 pieces. Roll into balls and make a hole in the middle of each with the finger.
Fill with about one teaspoon jam and pinch closed.
Cover with oiled plastic wrap and allow to rise in a warm place for about 20 minutes or until double in size.
Deep-fry in hot oil until golden brown.
Drain on kitchen paper and toss in castor sugar.
To make ring doughnuts, cut out rounds from centre.
Fry small round cuts and coat with glace icing or melted chocolate, or decorate with hundreds and thousands.
Fry the doughnuts first, make a hole in the centre with the finger and then fill with jam.