Preheat oven to 160C (350F/gas4). Melt chocolate with 2 tablespoons rum. Cream butter and sugar until pale and fluffy. Beat in 3 egg yolks. Whisk 3 egg whites with a pinch of salt and 1 tablespoon of castor sugar until very stiff.
Stir the melted chocolate into the butter and sugar mixture followed by the ground almonds. Fold in one third of the egg white then the flour. Fold in remaining egg white. Divide the mixture between two 8inch sandwich tins. Important! Cooking time about 18-22 minutes. Cake should be slightly underdone in centre. Side should be cooked but centre still slightly unset.
Melt chocolate with rum. Beat in butter and allow to cool until spreading consistency. Sandwich cakes with one third of the icing using the remainder to cover the top and sides. Decorate with flaked almonds.