First butter the ramekins and dust well with flour. Put the chocolate into bowl over simmering water (do not let the water touch the bowl). Add the rum and butter, melt over a low heat. Remove once melted and then allow to cool to room temperature, now add the egg yolks.
Now put the egg whites into a mixing bowl and beat the egg whites to a soft peek. Reduce the speed and add the sugar and flour. Mix until well incorporated. Now fold in the cooled chocolate. Add to the prepared ramekins ¾ full and cook in the oven at 200C for 10 minutes approximately or until firm to the touch. Turn out and eat while still warm.