Grease a nonstick baking paper and line a 23x33cm Swiss roll tin. Preheat the oven to Gas 4. 180讈. fan 160讈. Melt the chocolate over a pan of gently simmering water. Place the egg whites in a large grease-free bowl and using an electric whisk. whisk them until they form soft peaks. Set aside. Place the egg yolks and sugar in a large bowl and stand it over a pan of simmering water. Whisk until the eggs are fluffy and thick enough to leave a trail when the whisk heads are lifted from the mixture. Stir in the chocolate then gently fold in the egg whites. Pour into the prepared tin and bake for 20-25mins or until the centre springs back when lightly touched with your finger. Whilst the cake cooks. lay a sheet of nonstick baking paper out on the work surface and sprinkle with a little caster sugar. Tip the roulade out onto the paper. carefully peel away the lining paper. trim the edges then cover the roulade with a clean. damp tea towel. Leave to cool. Whip the cream and icing sugar until it forms soft peaks. gently fold in half the raspberries. Spread the cream mixture over the roulade. scatter over the remaining raspberries. then starting from one of the narrow ends. carefully roll up the roulade using the paper to help. Transfer to a serving plate and dust with a mixture of icing sugar and cocoa.