Preheat the oven to 180C/Gas 6 and butter six 150ml ramekins; melt the chocolate in the microwave
Place the butter, sugar, eggs and flour in a food processor and whizz to a smooth batter, then add the melted chocolate and whizz again; finally add the rum
Divide the mixture between the ramekins, dust with a little more sugar and place on a baking sheet and bake for 9 minutes until just starting to rise; there should be a thin rim of cooked cake on the outside and a sticky river of molten gooey chocolate inside
Serve immediately with ice cream or crème fraiche; the cake mixture can also be prepared several hours in advance, in which case cover and chill the ramekins and bake for 11-12 minutes
6 servings