Break the chocolate up in a non-stick pan and place it on a low heat until it completely melts. Add a few drops of chocolate extract (if using) and set aside to cool.
In a clean bowl, whip the cream and sweetener to semi-soft peaks using a hand mixer.
Once the chocolate has cooled, fold it into the whipped cream until incorporated. The mixture will stiffen quickly, so spoon or pipe it immediately into your serving glasses or bowls.
If you choose to make pretty decorations, simply whip together the cream and sweetener to stiff peaks, then spoon or pipe them over the chocolate mousse. Grate over any chocolate you may have leftover to finish beautifully.
These can be made ahead of time (kept covered in the fridge), but I suggest that you to remove it from the fridge about 30 minutes before eating.
Notes
Tips:
Allow the melted chocolate to cool before incorporating it into the whipped cream to avoid a grainy mixture. By this, I simply mean to cool to room temperature: do not allow it to harden again.
When folding the melted chocolate into the whipped cream, do so in large circular motions ensuring there is still plenty of air incorporated into the mousse.