Finely chop the dark chocolate and place it in a heatproof bowl.
Heat ½ cup of the heavy cream until just simmering.
Pour the hot cream over the chocolate.
Let it sit for 2 minutes.
Stir until the chocolate is completely smooth.
Allow the chocolate mixture to cool to room temperature.
In a separate bowl, whip the remaining 1 cup heavy cream with the powdered sugar until soft peaks form.
Gently fold one-third of the whipped cream into the chocolate mixture.
Fold in the remaining whipped cream until fully combined.
Divide the mousse evenly into serving glasses.
Refrigerate for at least 2 hours before serving.
Garnish with chocolate shavings or fresh berries if desired.
