The number of sponge fingers you need depends on the shape of your serving dish.
100 g (4 oz) plain chocolate, broken into pieces
1 tablespoon water few drops of vanilla flavouring
3 eggs, separated
20-24 sponge finger biscuits
chocolate curls and orange slices, to decorate (optional)
1. Put the chocolate, water and vanilla flavouring into a heatproof bowl. Set the bowl over a pan half full of simmering water and leave, stirring occasionally, until the chocolate is melted. Set aside to cool slightly.
2. In a large bowl, whisk the egg
yolks until pale. Add the melted chocolate and continue whisking until the mixture is thick.
3. In a clean, dry bowl and using clean beaters, whisk the egg whites until standing in stiff peaks. Fold the egg whites into the chocolate mixture with a large metal spoon.
4. Use half the biscuits to line the base of an oblong 850 ml (1 1/2 pint)
serving dish. (Trim the biscuits, if necessary, so they fit neatly.)
5. Pour the chocolate mixture over the biscuits, then cover with cling film; refrigerate at least 2 hours.
6. To serve: arrange the remaining biscuits, cut in half, on top of the mousse. Decorate with chocolate flakes and orange slices, if liked, and serve chilled.