Add two cups of water to a sauce pan and turn the heat to high. Place a heat proof bowl on top of the pan.
Break up the chocolate and place it in the heat proof bowl with the cream and melt it until smooth.
Beat the egg yolk in a bowl, and add a bit of the melted chocolate to temper it.
Pour the tempered egg yolk back into the melted chocolate.
Stir until thickened, approximately 2 minutes.
Remove the heat proof bowl from the stove and leave it aside to cool.
While the chocolate is cooling, whip the cream in the stand mixer with sugar until soft peaks form.
Fold the whipping cream into the chocolate mixture and refrigerate for at least one hour while the cakes are cooling.