First, make the ganache filling. Gently melt the chocolate in a microwave or in a heatproof bowl set over a saucepan of simmering water. Stir the melted chocolate into the crème fraîche, adding the pulp from 1½ passionfruit. Cover and chill for at least 3 hours, or overnight, to firm up.
Preheat the oven to gas 4, 180°C, fan 160°C and line a 20x30cm (8x12in) Swiss roll tin with nonstick baking paper.
Whisk the egg yolks and 75g (3oz) caster sugar together until pale and mousse-like. In a separate bowl, whisk the egg whites until they form soft peaks. Add the remaining caster sugar and whisk until thick and glossy. The whites should stand in stiff peaks. Fold the ground almonds and a pinch of salt into the egg yolk mix and fold in the whites. Pour into the tin and smooth out. Bake for 10-12 minutes, until golden.
Sprinkle another sheet of nonstick baking paper with a little caster sugar and turn the cooked sponge out onto it, peeling its paper away. Trim the sponge to leave a clean edge all the way around. Roll up from one long side, including the paper in the roll. Leave to cool.
Carefully unroll the sponge. Spread with the ganache, leaving a gap around the outside. Spoon the remaining passionfruit pulp over and re-roll the roulade. Transfer to a plate. Use a fine sieve to dust over the icing sugar, then decorate with white chocolate curls.
To make chocolate curls, chill a large baking sheet in the freezer. Melt 200g (7oz) white chocolate (as described in step 1, left)and pour onto the sheet in a 1cm (½in) layer. When it sets, use a large knife, held at an angle away from your body, to scrape away from you, pushing the chocolate into curls. They can be kept in the fridge for a few days, or in the freezer for a few months.