Preheat oven to 180 degrees C and grease a baking tray.
In a mixing bowl, beat butter, 2 eggs and brown sugar until fluffy. Grate the chocolate finely and add to the mixture along with the flour, baking powder, cinnamon, rum and cloves. Mix well then fold in almonds. The dough should be quite firm.
Spread the dough out to a thickness of about 7mm on the baking tray. This is best done with a spatula that has been dipped in water. Whisk the egg white and spread over the dough.
Bake 20-25 minutes or until it tests clean with a toothpick. Carefully remove the cake to a wire rack to cool. Once cool enough to handle, place on a cutting board.
Mix the icing sugar and 3 tablespoons water and brush on while the dough is still warm. Immediately cut into bars and leave to set. Bars will keep in a tin up to 1 month.