First of all, pop the baking tray into the oven to pre-heat.
Now for the soufflé, break the chocolate into a heatproof mixing bowl and add cream, and place the bowl over a pan of barely simmering water, making sure the base of the bowl doesn't touch the water.
Leave it until the chocolate is just soft, which will take about 6 minutes, then remove it from the heat. Now whisk the rum into the melted chocolate mixture and allow it to cool.
In the meantime, brush the soufflé dish with melted butter and dust with caster sugar.
Now in a small bowl whisk the egg yolks and then whisk them into the chocolate mixture. In a large, grease-free bowl - and making sure the beaters of your electric hand whisk are clean and dry - whisk the egg whites until they form soft peaks. Then whisk in the caster sugar.
Now in another large bowl, using a large metal kitchen spoon, mix a quarter of the egg white into the chocolate mixture to loosen it then fold in the rest gently and carefully.
Next, pour the whole lot into the soufflé dish and if there are any patches of white blend them in with the tip of a skewer and bake on the pre-heated baking tray near the centre of the oven for about 20 minutes, or until the soufflé is puffy and springy to the touch.
Meanwhile, make the chocolate sauce.
Break the chocolate into another heatproof mixing bowl and add the cream. Place it over a pan of barely simmering water, once again making sure the base of the bowl doesn't touch the water. After about 6 minutes, remove it from the heat and beat it with a wooden spoon until smooth.
Pour the sauce into a small jug and keep warm.
Serve the soufflé straight from the oven, dusted with icing sugar. Hand around the chocolate sauce and chilled pouring cream separately.