Dark chocolate and butter should be combined in a heat-resistant bowl. Ensure the bowl's bottom doesn't touch the water as you place it over a saucepan of heating water. Chocolate and butter must be smooth and melted after stirring.
Remove it from heat and let it cool just a little.
Whisk the egg yolks, sugar, and vanilla extract in another dish until they are creamy and light yellow.
Now gradually add the melted chocolate to the egg yolk mixture while stirring regularly to ensure complete integration.
Whip the heavy cream in another bowl until it forms soft peaks.
In a bowl, whip the egg whites until firm peaks form.
Avoid overmixing as you slowly fold the whipped cream, beaten egg whites, and chocolate mixture into one another.
Spoon the chocolate mousse into serving glasses or bowls for at least two hours or until set, cover, and chill.
Fresh berries should be garnished before serving.