It's worth choosing the best chocolate you can: look for Valrhona, Menier, Suchard, Lesme or Callebaut. If you can stop yourself from finishing this in one sitting, this cake keeps well.
2 x 18cm/7in sandwich tins
Preheat the oven to 180C/350F/Gas 4.
Line the base of each of the tins with a round of greaseproof paper. Brush the bottom and sides with melted butter and dust with a little rice flour.
Melt the chocolate with the rum in a heatproof bowl over a pan of simmering water or in a low oven.
Bring a small saucepan of water to the boil and add the almonds. Bring back to the boil for 2-3 minutes and then test an almond to see if the skin is loose. Drain the almonds, peel and discard the skins. Grind the whole almonds in a food processor until they are slightly gritty.
Cream the butter and add the sugar. Beat until pale, light and soft. Beat in the egg yolks, one by one. Whisk the egg whites until stiff. Add the extra tablespoon of sugar and continue to whisk until stiff peaks form, then add the melted chocolate to the butter and sugar mixture. Divide the prepared almonds into four portions. Add one portion to the creamed mixture. Fold in a quarter of the egg white, followed by more almonds. Fold in the remaining eggs and almonds alternately until they have all been added.
Divide the mixture between the two prepared tins and make a hollow in the centre of each cake. Bake in the oven for 20-25 minutes. The sides should be cooked but the centre still a little unset. Leave to cool for a few minutes in the tins, turn out gently onto a wire rack, remove the paper and allow to cool completely.
To make the icing, melt the chocolate with the rum in a heatproof bowl over a pan of simmering water or in a low oven. Whisk in the butter, a tablespoon at a time, until melted. Remove from the heat and whisk occasionally until cool. If the icing seems too runny, put the bowl in the fridge and allow to firm up. Whisk to lighten and then use.
When the cake is completely cold, fill and ice with the chocolate mixture. (If you want to cover the sides and pipe a border around the top, make 1½ times the quantity of icing.) Decorate with flaked almonds and crystallised violets.