Put the butter or margarine, sugar, flour, cocoa and eggs into a mixing bowl or food processor and beat together until smooth. Divide the mixture between 6 greased sections of a deep muffin tin, then press 2 squares of dark chocolate into each and cover with the pudding mixture.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 18–20 minutes until well risen, slightly crusty around the edges and the centre springs back when pressed with a fingertip.
Meanwhile, warm the white chocolate, cream and vanilla essence together in a small saucepan, stirring until the chocolate has completely melted.
Loosen the edges of the baked puddings with a round-bladed knife, then turn out and transfer to shallow serving bowls. Drizzle with the white chocolate cream and serve immediately.