Melt the dark chocolate in a double boiler until completely smooth.

In a small saucepan, combine the heavy cream and sugar, and heat until the mixture begins to gently boil.

Mix the egg yolks in a bowl, gradually adding half of the hot syrup while whisking continuously. Stir in the remaining syrup and heat on low, then remove from heat once steam rises.

Transfer the yolk mixture into a bowl and whip at high speed until it cools to room temperature.

Fold half of the yolk mixture into the warm melted chocolate until well combined, then add the rest and fold gently.

Whip the cold heavy cream until soft peaks form, then fold half into the chocolate mixture, followed by the remaining whipped cream.

Pour the mousse into individual glasses or molds and refrigerate for a few hours or overnight.