Melt the chocolate and rum or coffee in a pot set over simmering water, stirring to combine. Cream the butter and 2/3 cup (160 ml) sugar together until pale yellow and fluffy. Beat in the egg yolks. In a separate bowl beat the egg whites and 1 tablespoon (15 ml) sugar until stiff. Combine the chocolate mixture, butter mixture, ground almonds, almond extract, and blend thoroughly.
Fold 1/4 of the egg whites into the chocolate mixture, followed by 1/4 of the sifted flour. Repeat until all the egg whites and flour have been incorporated. Pour the batter into a greased and floured 8-inch (20 cm) round cake pan, pushing the batter to the edges of the pan with a spatula. Bake in the center of a preheated 350F (180C) oven for about 25 minutes.
When done, the cake will have puffed up, the outer edges of the cake should be firm, and the center should move slightly when the pan is shaken. A toothpick inserted in the outer portion should come out clean, and slightly oily when inserted in the center. Allow the cake to cool in the pan for 10 minutes. Run the blade of a knife around the inside of the pan and invert the cake onto a cooling rack. Allow to cool completely before icing.
To prepare the icing, melt the chocolate and rum or coffee in a pot set over simmering water, stirring to combine. Remove from the heat and beat in the butter 1 tablespoon (15 ml) at a time. Place the pot in a large bowl filled with ice water and continue beating until the mixture has cooled to spreading consistency. Spread the icing over the cake with a knife or spatula and decorate with whole almonds.