With a hand-held electric whisk, beat egg whites until they form soft peaks, then whisk in the caster sugar a little at a time. Fold in the ground hazelnuts  and spread the mixture evenly in the tin. Bake in centre of oven for 20min until lightly golden. Cool completely in the tin. Turn out on to a piece of greaseproof paper slightly larger than the roulade, lightly dusted with icing sugar. Gently ease away the lining paper. Melt the chocolate in a heatproof bowl over simmering water. Remove from the heat and leave to cool. Whip the cream until it just holds its shape, and divide between two bowls. Reserve 4tbsp of the melted chocolate for decoration, and fold the rest into one bowl of whipped cream until evenly mixed and mousselike. Smooth this evenly over the roulade, leaving a 1cm (½in) border.  Spread the rest of the whipped cream over the chocolate mixture, and then roll up the meringue to form a long log shape – a few cracks are normal, so don’t worry!