Melt the chocolate in a heatproof bowl set over a pan of simmering water (do not allow the base of the bowl to touch the water).
Whisk the egg whites and lemon juice in a large, clean bowl until they form soft peaks. The lemon juice will stabilise the egg whites, make them easier to work with and help to prevent over-whisking.
Add the sugar and continue to whisk until firm peaks form when the whisk is removed. Do not whisk beyond this stage - the egg whites will start to collapse and separate into dry froth and runny liquid, and you'll lose all the air that you've whisked in.
When the chocolate has melted, remove the bowl from the heat. Whisk one-third of the egg whites into the hot chocolate quickly and vigorously, until thick and well combined - if you add the egg whites in too slowly, their cold temperature can make the hot chocolate seize, solidify and result in a lumpy mousse.
Fold the remaining egg whites into the chocolate mixture, using a spatula, until all of the egg white has been completely incorporated into the chocolate. Don't overmix at this stage as you'll knock out the air bubbles and the mousse will be dense.
Spoon the mousse mixture into four Martini glasses. Chill in the fridge for 2-3 hours, or until set.
Alternatively, use the mousse to make a hot chocolate soup. Preheat the oven to 180C/350F/Gas 4. Divide the mousse among four small ovenproof pudding bowls or ramekins. Place the pudding bowls into the oven and bake the chocolate mousses for 3-5 minutes, or until puffed slightly and warmed through.