Chop the chocolate into small, even pieces.
Place it in a heatproof bowl with butter.
Melt everything over simmering water gently.
Stir until completely smooth and glossy.
Let it cool for five minutes.
Add egg yolks to the cooled chocolate mixture.
Whisk them together until fully combined.
The mixture should look thick and shiny.
Set this bowl aside for now.
Whip egg whites in a clean bowl.
Beat until soft peaks start forming.
Add sugar gradually while continuing to beat.
Keep going until stiff peaks appear clearly.
Pour cold cream into another clean bowl.
Add vanilla extract to the cream mixture.
Beat until soft peaks form in cream.
Don't over-whip or it becomes butter.
Gently fold egg whites into chocolate mixture first.
Use a rubber spatula with gentle motions.
Then fold in the whipped cream carefully.
Keep the mixture light and airy throughout.
Divide mousse into six individual serving glasses.
Cover them with plastic wrap tightly.
Refrigerate for at least two hours minimum.
Serve cold with fresh berries if desired.