Dice the chilled butter. Do not return to the refrigerator. Sift the flour with the pinch of salt and the saffron powder.
Put some water in saucepan of a size that will allow a large heatproof mixing bowl to sit on top of it. Bring to the boil, then lower to a simmer.
Drain the sultanas and toss in plain flour to coat evenly and reserve.
Using an electric hand-held beater, whisk the eggs and sugar in the mixing bowl. Then place this over the saucepan of barely simmering water and continue to whisk for 10 minutes, until the mixture is thick and drops from the whisk in ribbons.
Add about one-third of the flour and fold in with a spatula. Then fold in one-third of the diced butter. Repeat this two-stage process three times, alternating flour then butter, stirring and folding until you have a smooth mass with the butter completely incorporated.
Finally fold in the sultanas and pour the batter into the lined Swiss roll tin.