Start off by setting a small basin over a small pan of barely simmering water, without the bowl touching the water, then add the coffee granules to the basin.
Measure in 1 tablespoon hot water and stir to dissolve the granules. Now break up the chocolate into small pieces, add these to the basin and stir till the chocolate has melted and been incorporated with the coffee and you have a smooth sauce.
Next separate the egg, the white into a medium-sized bowl and the yolk into a smaller one. Beat up the yolk then stir it well into the chocolate mixture, off the heat. Now stir the rum into it, and leave to cool for about 10 minutes.
Meanwhile, whisk up the egg white to the soft-peak stage (that is, when it leaves soft peaks when you lift the whisk out of it). Then, using a metal spoon, fold the egg white into the chocolate mixture. Spoon the whole lot into a stemmed wine glass, cover with clingfilm and chill for a couple of hours.
To serve: combine the cream with the dissolved coffee, pour this over the top of the mousse and sprinkle with a little grated chocolate.