To make the fruit mix, put the sultanas and currants in a mixing bowl. Pour over the boiling water and set aside to cool. Cover with plastic wrap and place in the refrigerator overnight, or for up to 2–3 days. Drain the fruit and add the brown sugar and mixed spice, stirring well to combine. Set aside.
Take the rested dough from the refrigerator and roll it out into a rectangle, about 35 x 100 cm (14 x 39½ inches) and about 5 mm (¼ inch) thick. Rolling the dough may take quite a lot of energy as it may keep springing back to its original size. If this happens, rest the dough in the refrigerator for about 10 minutes then resume rolling. This resting process may need to be done a number of times. If the dough is becoming too large to fit in the refrigerator simply fold it over and place on a tray before placing it in.
Cut the dough in half to make two 35 x 50 cm (14 x 20 inch) rectangles. Place the dough on trays lined with baking paper and place in the refrigerator for about 10 minutes.
Lay one sheet of dough on a lightly floured surface, with the short length running parallel to the edge of the bench. Spread ½ cup of crème pâtissière evenly over the rectangle, spreading all the way to the edges. Top the rectangle with half of the fruit mix to evenly cover the crème pâtissière. Starting with the short edge furthest away from you, firmly and tightly roll the dough towards you, as if rolling a newspaper for delivery. Wrap the log in baking paper and place in the refrigerator for about 20 minutes to firm up. Repeat with the remaining sheet of dough.
Remove the baking paper from both logs and place the dough on a clean work surface. Cut each log into slices, about 1.5 cm (⅝ inch) wide, to make about 30 rounds in total.
Preheat the oven to 240°C (475°F/Gas 8). Place the rolls back onto the lined trays at well-spaced intervals. Cover loosely with a damp tea towel (dish towel). Set aside in a warm room (about 25°C–27°C/77°F–81°F) for 1½–2 hours, or until almost doubled in size. Spray the tea towel with water occasionally if it becomes dry.
Remove the tea towel, brush the top of each pan au raisin lightly with egg wash and place in the oven. Reduce the oven temperature to 190°C (375°F/Gas 5) and bake for about 15 minutes, or until a deep golden colour. Cool slightly on the trays before dusting with icing sugar and serving.