Preheat the oven to 400°F (200°C). Butter four 8 oz (200 ml) ramekins well and place on a baking sheet. Combine the chocolate and butter in a large heatproof bowl set over a pan of very hot water and stir until smooth, then set aside to cool for 15 minutes.
Mix in the sugar, then the eggs, one at a time, followed by the vanilla extract, and finally the flour. Divide the mixture evenly among the ramekins. Bake for 10-12 minutes or until the edges are set and the tops are firm to the touch, but the middles are still soft.
Carefully run a knife around the edge of each pudding, then invert onto individual serving plates and serve with whipped cream.
Assemble these the day before, cover, and refrigerate until ready to bake. Don't overcook the puddings or the middles will be cakey rather than gooey.