Combine the
hazelnuts and breadcrumbs on a baking sheet, then sprinkle over the
demerara sugar. Place under a medium grill and cook, stirring, until the
mixture is crisp and evenly browned. Leave to cool. Whisk the egg whites
in a bowl until stiff, then gradually whisk in the sugar until thick and
glossy. Whip the cream until if forms soft peaks and fold into the
meringue with the breadcrumb mixture and vanilla essence. Spoon the
mixture into a 1.2 liter loaf tin. Smooth the top level, then cover and
freeze for several hours, or until firm.
Meanwhile,
make the sauce. Strip the blackcurrants from their stalks using a fork
and put the blackcurrants in a small bowl with the sugar. Toss gently to
mix and leave for 30 minutes. Purée the blackberries in a blender or
food processor, then press through a nylon sieve until smooth. Add the
crème de cassis and chill well. To serve, turn out the ice cream on to a
plate and cut into slices. Arrange each slice on a serving plate, spoon
over a little sauce and decorate with fresh mint sprigs.