Place chocolate and butter in a microwave safe bowl and cook for 30 seconds. Remove from microwave and stir well.
Return chocolate to microwave for 15 seconds, remove and stir well. The goal is to get the chocolate just melted. If more time is needed, heat an additional 15 seconds, remove and stir well. Repeat until chocolate and butter are just melted. Stir until completely smooth.
Let cool slightly. The chocolate should not be so hot as to cook the egg yolk that you will be adding.
In a small bowl, combine egg yolks and mix well to combine.
Add egg yolks and vanilla to the chocolate mixture. Whisk together quickly to combine.
Place egg whites and 2 tbsp granulated sugar into a mixing bowl and beat well with an electric mixer until mixture forms stiff peaks, approximately 30-45 seconds.
Wash and completely dry the beaters.
In a separate bowl combine whipping cream and remaining 2 tbsp sugar and mix until soft peaks form, approximately 20-30 seconds.
Add a few spoonfuls of the egg white mixture to the chocolate mixture and GENTLY fold it in until throughly combined.
Fold in the whipped cream mixture until just combined.
Place in refrigerator to chill. At least two hours is ideal. Mousse is best consumed with in one day. Store in refrigerator.
Garnish with whipped cream and chopped chocolate if desired.