Generously butter a 12-cup bundt tin or two 20cm fluted ring tins. Sprinkle with flaked almonds, pressing them well into the butter to coat the bottom and sides of the tin. Put the boiling water, chocolate and bicarbonate of soda in a bowl and stir until smooth. Cream the butter and sugar until light and add the egg yolks one at a time, beating after each addition. Stir in the vanilla, then add the chocolate mixture a little at a time. Sift the flour, salt and baking powder and fold in alternately with the sour cream, mixing lightly until just combined. Beat the egg whites until stiff and fold into the creamed mixture with a large metal spoon. Turn gently into the prepared tin and bake in a preheated moderate oven 180C (350F) for 1 to 11/4hours for the large cake or 45 minutes for the small cakes, or until cooked when tested with a skewer. Leave in the tin for a minute, then turn out and cool on a wire rack.
Note: A bundt tin is used to give a pretty shape to continental cakes. It is a deep tin with fluted sides and a hole in the middle and should be available at kitchen shops and department stores.