Put the sieved flour into a mixing bowl, add a pinch of salt and, a little at a time, the Vin Santo and then the oil, stirring continually to avoid lumps forming. Now stir in the sugar, pine nuts, sultanas (which you have previously soaked in warm water for 10 minutes and then dried with kitchen paper) and the baking powder. Once everything is mixed together well, grease a baking tray with olive oil and dust it with a little flour. Shape the dough into walnut-sized balls and place them on the tray, leaving a bit of room between them. Bake in a preheated oven, at 160 °C, for about 15-20 minutes. If the biscuits seem to be browning too much, cover them with kitchen foil so that they can finish cooking without getting burnt. Remove them from the oven and, as soon as they have cooled a little, take them off the baking tray and sprinkle with plenty of icing sugar.