Melt together the chocolate, orange rind and juice and rum essence, if using, in a heatproof bowl set over a saucepan of gently simmering water.
Place the butter and all but 1 tablespoon of the sugar in a large mixing bowl and beat together until pale and creamy. Beat in the egg yolks, one at a time, then stir in the melted chocolate.
Whisk the egg whites in a large clean bowl until softly peaking. Add the remaining sugar, then whisk until stiff. Fold the egg whites into the chocolate mixture with the ground almonds, then spoon into 2 greased and base-lined 20 cm (8 inch) sandwich tins.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes, until the sides of the cakes are cooked but the centres are still a little unset. Remove from the oven, leave to cool in the tins for a few minutes then turn out gently on to a wire rack.
Ice the cake. Melt the chocolate as before then whisk in the butter, a tablespoon at a time, until melted. Remove from the heat and whisk occasionally until cool. If the icing is runny, put the bowl in the refrigerator until it firms up a little. Fill and ice the cooled cake with the chocolate mixture. Decorate with crystallized violet petals, if liked.