Melt chopped chocolate with 1/2 cup heavy cream in a heatproof bowl over simmering water or microwave in short intervals.
Stir until smooth and let cool slightly.
Whisk egg yolks into the warm chocolate mixture if using.
In a separate bowl, whip remaining 1 cup heavy cream with sugar, vanilla, and salt until soft peaks form.
Gently fold whipped cream into the cooled chocolate mixture in batches.
Spoon mousse into serving glasses or bowls.
Chill at least 2 hours until set and creamy.
Garnish with whipped cream, chocolate shavings, or berries before serving.