Instructions: Break the chocolate into little squares and place in a bowl over some gently simmering hot water to melt them. Do not overheat or you will coagulate the chocolate. Whisk half of the sugar with the egg yolks until smooth and creamy. Then, with a clean dry whisk, whisk the whites until stiff and foamy and then add the remaining sugar to the whites and continue to whisk together.

The secret here is to add the sugar towards the end, otherwise the whites will not stiffen. On the addition of the sugar the peaks will take on a glossy look and you should then add the rum. Now divide the egg whites between three separate dishes.

Add the butter to the melted chocolate and remove the pan from the heat. Now the timing is crucial - once cool ingredients are added to the chocolate, it will start to solidify so rapid whisking is required to avoid chocolate "chips".

First, fold the yolk mix into the first lot of whites and mix thoroughly but gently. Then add the chocolate mix quickly and fold thoroughly together to blend the chocolate.

Next, fold in the second batch of whites gently with a large spoon, and stir around. Then add the final batch, folding very gently to distribute the whites evenly and keep the mousse as light as possible. Work with upward movements to get as much air as possible into the bowl, and dont overwork each stage. Stop as soon as the whites are fully incorporated.

Spoon the mousse mixture quickly into serving dishes or ramekins and refrigerate for a couple of hours before serving. Remove the mousse from the fridge 15 minutes or so before serving.

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