Put the butter and sugar in a basin; if the former seems very hard warm it very slightly, taking care it does not get in the least oiled. Then beat them to a cream with a wooden spoon. Add the yolks of eggs, one by one, beating each in thoroughly. Then add the flour very lightly, and lastly the cleaned currants. Knead the mixture well, then roll it out to about an eighth of an inch thick. If the paste seems very soft, put it away in a cold place to harden it before rolling it out; in any case avoid adding extra flour.
Stamp the cakes in rounds the size of a breakfast cup or even larger. Put them on a slightly greased baking-tin and bake them slowly for ten to twelve minutes, until they are set and of a light yellow tint.
Just before they have finished cooking, whisk the white of egg slightly, brush each cake over with a little, then sprinkle over some castor sugar, put back in the oven, and finish cooking. These cakes keep well for some time in a tin.