Preheat the oven to 200°C, fan 180°C, gas 6. Grease a 33cm x 23cm swiss roll tin and line with baking parchment.
Melt the dark chocolate in a bowl over a pan of just simmering water, making sure the bowl doesn’t touch the water. Stir until smooth, then set aside.
In a large, clean bowl, whisk together the egg yolks and all but 1 tablespoon of the caster sugar until pale and thick. Mix in the melted chocolate and 1 tablespoon hot water until smooth.
In another clean bowl, whisk the egg whites until stiff, then whisk in the remaining 1 tablespoon caster sugar. Lightly fold into the chocolate mixture using a large metal spoon, then spoon into the prepared tin. Bake for 15-20 minutes, until risen and springy to the touch.
Cover with a sheet of baking paper and a damp tea towel. Leave for 15 minutes. Remove the tea towel and leave to stand until completely cold.
In a bowl, whip the cream, vanilla extract, and icing sugar until soft peaks form. Removed the paper and turn out the roulade onto a clean sheet of parchment. Spread with the cream to within 2cm of the edge. With the long side towards you and, using the paper to help, roll up the roulade.
Melt the white chocolate as in step 2. Using a teaspoon, drizzle the melted chocolate all over the roulade. Cut into slices to serve.
