Chill your tools. Place a mixing bowl and whisk (or beaters) in the fridge for 15 minutes. Cold equipment helps the cream whip faster and hold its shape.

Prep the chocolate. Finely chop the dark chocolate and place it in a heatproof bowl. Smaller pieces melt more evenly and prevent scorching.

Warm a portion of the cream. In a small saucepan, gently heat 1/2 cup of the heavy cream until it’s steaming but not boiling. Remove from heat.

Make a quick ganache. Pour the hot cream over the chopped chocolate. Let it sit for 1 minute, then stir slowly until smooth and glossy. If you like, stir in a pinch of salt or vanilla. Let the mixture cool to room temperature; it should be fluid but not warm.

Whip the remaining cream. In your chilled bowl, add the remaining 1 cup of heavy cream and the sugar. Whip to soft-to-medium peaks. The cream should be billowy and hold gentle peaks that slightly droop. Don’t go all the way to stiff peaks or it may turn grainy when folded.

Lighten the chocolate. Add a heaping spoonful of whipped cream to the chocolate ganache and stir to loosen it. This step makes folding easier and prevents deflation.

Fold carefully. Add the remaining whipped cream in two to three additions, using a spatula to fold gently from the bottom up. Rotate the bowl as you go. Stop as soon as there are no streaks. Overmixing can knock out the air.

Portion and chill. Spoon the mousse into serving glasses, ramekins, or a larger bowl. Smooth the tops if you like. Chill for at least 1–2 hours until set and lightly firm.

Garnish and serve. Top with extra whipped cream, shaved chocolate, or berries. Serve chilled and enjoy the silky texture and deep chocolate flavor.