Bring the Recioto, salt and butter to the boil in a saucepan; blend the flour in with a wooden spoon, stirring briskly to avoid lumps.
When the mixture begins to firm up and come away from the sides of the pan to form a ball, take the pan off the heat and tip the ball into a wide dish; leave to cool for 20 minutes.
Use a spatula to blend in the eggs one at a time – don’t start with another egg until the previous one is fully blended – and when the mixture is soft and smooth, add the diced apples and sultanas.
Bring the oil to a boil (175° C) in a deep-sided pan. Drop balls of the mixture into the hot oil using two spoons, one to scrape the mixture off the other.
Fry the balls in the oil, turning them over gently from time to time, until all surfaces have taken on a golden colour. Drain the balls when done on kitchen paper.
Sprinkle with caster sugar and serve.