I've been making this mousse for many years and always get lots of compliments whenever I serve it. The secret is to use the best chocolate you can find (definitely no cooking chocolate). Instead of using dark chocolate only, you can use a mixture of milk and dark chocolate. You'll get a paler as well as slightly sweeter mousse.
You will need:
4 very fresh egg yolks, preferably organic
300 g (11 oz) good dark eating chocolate, I used dark Toblerone
4 Tbsp fine sugar
2 Tbsp rum, brandy, whiskey or any other booze you might like (optional)
500 ml (17 fl.oz) single cream (very cold)
(serves 8 to 10)
First beat the cold single cream until soft peaks form. Set aside.
Break chocolate into smallish pieces and melt either in microwave or in a double boiler (in a bowl over a pan of simmering water). In the meantime put egg yolks in a bowl together with sugar and beat with an electric mixer until pale and light. It is important that the egg yolks are at room temperature. Then add alcohol (if using) and mix well. Once the chocolate has melted, let cool slightly, then mix into the egg mixture.
Now fold the chocolate/egg mixture into the cream with a rubber spatula or a big metal spoon making sure not to over mix.
Place mousse into a clean bowl, or into small individual serving dishes, and chill in the fridge for a few hours or over night.
Scoop out mousse with a spoon dipped in hot water first. 
Enjoy!
If I have leftovers of this delicious mousse I freeze it to be enjoyed later on as a creamy chocolate ice cream.
Bon Appétit
Vreni
