Place the chopped chocolate in a heatproof bowl.
Heat 120 ml (½ cup) heavy whipping cream in a small saucepan until it just comes to a boil. Pour the hot cream over the chocolate. Let it sit for a minute, then stir with a spatula until smooth and glossy.
Alternatively, combine the chocolate and cream in a heatproof bowl and microwave for approximately 1 minute. Stir until smooth, adding more time if needed.
Set the ganache aside.
In a separate bowl, whip the remaining 360 ml (1½ cups) cold heavy whipping cream at medium speed until soft peaks form.
Fold approximately one-quarter of the whipped cream into the ganache using a spatula.
Gently fold in the remaining whipped cream until evenly combined. Do not overmix otherwise the mousse will appear grainy.
Transfer the mousse to a piping bag or use a spoon to portion it into individual serving cups.
Refrigerate for 2–3 hours, or until set.