Ingredients
50g/2oz plain flour
60g/2½oz ground almonds
225g/8oz good quality dark chocolate (at least 70 per cent cocoa solids), broken into pieces
225g/8oz butter
6 free-range eggs, separated
50g/2oz soft brown sugar
grated orange zest (optional)
175g/6oz caster sugar
icing sugar, for dusting

Method
1. Preheat the oven to 180C/350F/Gas 4.
2. Grease and line the base and sides of a 20cm/8in loose-bottomed or clip-sided cake tin with greaseproof paper.
3. Sift the flour and almonds into a large bowl.
4. Place the chocolate pieces and butter into a heatproof bowl set over a pan of gently simmering water, making sure the water doesn't touch the base of the bowl. Heat, stirring from time to time, until melted and smooth. Remove from the heat and allow to cool slightly.
5. Separate the eggs into two large bowls. Add the brown sugar to the bowl with the egg yolks and whisk together for a few minutes until frothy. Add the flour mixture, melted chocolate and grated orange zest, if using. Fold lightly to mix, using a large metal spoon.
6. Whisk the egg whites until soft peaks form when the whisk is removed. Gradually add the caster sugar and continue whisking until stiff peaks form when the whisk is removed.
7. Gently fold the egg white mixture into the chocolate mixture. Pour the mixture into the cake tin.
8. Bake in the oven for 45-50 minutes, or until crisp on top and squidgy inside. Test the cake with a cocktail stick or skewer - it should come out a bit sticky, but not wet. Allow to cool on a wire rack - the cake will sink but that's ok. Carefully peel the paper off when the cake's cold.
9. To serve, dust the cake with icing sugar and cut into slices.