Preheat oven to 100 degrees C. Line a baking tray with baking paper. Slightly warm the milk with the butter.
In a mixing bowl, combine flour, yeast, sugar, lemon sugar, cardamom, nutmeg, cinnamon, salt and currants and mix well.
Add the warm milk mixture, the egg white and half the yolk. Reserve the other half of the yolk in a cup.
Knead the dough until smooth on a floured work surface. Roll into a sausage and cut into 12 pieces. Roll each pieces into a smooth ball. Place on the baking tray and with a good sharp knife cut a 1cm deep cross in the top of each. Mix remaining yolk with 1 tablespoon milk and brush the rolls with it.
Place in the oven and let rise for 30 minutes, then turn to 200 degrees C and bake 25 minutes. Turn off the oven and let bake 5 minutes more.
Cool buns on a wire rack. Combine the icing sugar with the milk and fill the crosses with the icing.
To make lemon sugar, combine the zest of 3 unsprayed lemons with 1 cup white sugar. Keeps in the fridge up to 4 weeks.