Whip heavy cream to medium peaks and place in refrigerator.
Melt chocolate and allow to cool to just above body temperature. It should feel warm to the touch.
In the bowl of a stand mixer, fitted with the whisk attachment, beat yolks and sugar until tripled in volume and pale yellow. Add chocolate and beat well. This with thicken considerably. If it looks like it is breaking, just keep whisking. Move quickly to ensure it doesn’t continue to thicken.
Sacrifice some whipped cream by adding a small amount of whipped cream to the chocolate mixture. Fold aggressively with the whisk to incorporate. Quickly then fold the chocolate into the rest of the whipped cream. Spoon immediately into containers. You can also pipe the mousse, but the resulting mousse will be a bit denser.