In 1/2 cup of boiling water, dissolve the cocoa, add the sugar, egg yolk and butter. Beat all of these products until you get a smooth, light cream. Add sugar if desired.
Dip the biscuits in rum and arrange them next to each other on baking paper in a rectangular baking dish. Next, starting stacking up. Smear cream on the first layer of biscuits, then cream again, then biscuits etc., until you've used four fifths of the total cream.
Put the cake in the refrigerator to set. Shortly before serving, smear the rest of the cream on the very top.
You can sprinkle powdered cocoa on the top as well.