Break the dark chocolate into small pieces and melt it slowly over a double boiler or carefully in short bursts in the microwave. Once melted, let it cool until it’s just warm to the touch—too hot will affect the cream later.
Pour the chilled heavy cream into a clean bowl and whip it gently until it forms soft peaks. If you want a sweeter mousse, sprinkle in the powdered sugar while whipping. Be careful not to overbeat; the cream should still feel light and airy.
Slowly fold the cooled chocolate into the whipped cream using a spatula. Use gentle, sweeping motions to combine them evenly without losing the fluffy texture. Work in batches for the best results.
Divide the mousse evenly into serving glasses or ramekins. Cover and chill in the fridge for at least one hour to let it set and develop its signature silky texture. Serve chilled and enjoy!