Butter lightly 4 small soufflé dishes, 10 cm (4 inches) in diameter and 6 cm (2½ inches) deep, then swirl 1 teaspoon caster sugar around each dish and shake off any excess. Stand the dishes on a baking tray.
Beat half the remaining caster sugar and the egg yolks in a bowl until thick and mousse-like. Sift the flour over the surface, then gently fold it in.
Bring the milk just to the boil in a pan, then gradually whisk it into the egg yolk mixture. Return the milk to the pan and cook over a medium heat, stirring until thickened and smooth. Remove the pan from the heat, add half the white chocolate and stir until melted. Mix in the vanilla essence, cover and leave to cool.
Whisk the egg whites into stiff peaks. Gradually whisk in the remaining caster sugar until thick and glossy. Fold a large spoonful of egg whites into the cooled milk mixture to loosen it, then fold in the remaining white chocolate. Gently fold in the remaining egg whites.
Spoon into the soufflé dishes and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10–12 minutes until the soufflés are well risen.
Heat all the sauce ingredients gently in a pan, stirring until smooth, then pour into a serving jug. Dust the tops of the soufflés with chocolate powder and icing sugar and serve immediately drizzled with the sauce.