From the fantastic book Great GF Baking, by Louise Blair
grated rind & juice of 1 orange
few drops rum essence (optional)
150g unsalted butter, softened
150g caster sugar
4 eggs, separated
150g ground almonds
100g unsalted butter
Grease 2 x 20cm sandwich cake tins. Preheat oven to 180 °C. Melt together chocolate, orange rind and juice and rum essence in a heatproof bowl over a pan of simmering water.
Place butter and all but 1 tablespoon of sugar in a large bowl and beat until pale and fluffy. Beat in the egg yolks one at a time then stir in melted chocolate.
Place egg whites in a large, clean bowl and whisk until they form soft peaks. Add the remaining sugar and continue to whisk until stiff peaks form. Fold the egg whites into the chocolate mixture with the ground almonds, then spoon into the prepared cake tins.
Place in oven for 20-25mins until the sides are cooked but the centre is still a little unset. Remove the cakes from the oven, leave to cool for a few minutes in tins then turn out gently onto a wire rack.
To ice, melt chocolate then whisk in the butter, a tablespoon at a time until melted. Remove from 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.