A chocolate marquise can either be chilled or frozen but I think this luxurious concoction is best as a glorious ice cream. As it is a pudding which can be made well ahead and is extremely impressive (though not difficult) it is ideal for a special dinner party. The frozen marquise is sliced across in thick slices to serve, and if you like accompany it with cream or soured cream to spoon either over or underneath the slices. To turn out the marquise for serving rub the outside of the tin with a hot cloth, then slip a warm palette knife carefully between the biscuits and the tin. Invert on to a serving dish, giving a good shake.
The quantity here serves 6. Use about 25—30 sponge fingers or boudoir biscuits, 75g (3oz) granulated sugar, 6tablespoonsbrandy or rum, 2tablespoonswater, 125g (4oz) plain chocolate, 1tablespoontop-of-the-milk, 1 sachet powdered gelatine, 3 large eggs (size 2), separated, 25g (1oz) caster sugar and 175ml (6fl oz) double cream plus extra chocolate for decoration.
Lay the sponge fingers in a shallow dish. Dissolve the sugar in the brandy or rum and water over a low heat, then bubble for a second or two before spooning over the sponge fingers. Leave for 30 minutes or more until the syrup is absorbed, carefully turning over each sponge finger once.
Line the base of a 1kg (2lb) loaf tin with a piece of baking parchment cut to fit. Line the base and sides of the tin with the soaked sponge fingers, cutting off neatly along the top of the tin with a sharp knife. Use the cut-off sponge finger tops to fill in any gaps.
Melt the chocolate and top-of-the-milk in a heatproof bowl set over a pan of gently simmering water. Dissolve the gelatine in about 3 tablespoons hot water, then stir it into the melted chocolate. Set the mixture aside to cool. Beat the egg yolks with the caster sugar until pale and thick, then beat in the slightly cooled chocolate.
Whip the cream until thick, and in a second bowl whisk the egg whites until soft peaks form. Fold the cream into the chocolate mixture, followed by the egg whites. Spoon into the biscuit-lined tin, smooth the top with a knife and freeze for several hours until firm. Unmould (above) on to a serving plate and top with grated chocolate, if you like.