Place the chopped dark chocolate in a heatproof bowl.
Fill a saucepan halfway with water and bring it to a gentle simmer. Place the bowl over the saucepan, making sure the bottom of the bowl does not touch the water.
Stir until the chocolate is completely melted, then remove the bowl from the heat and let the chocolate cool slightly.
In a separate bowl, beat the eggs and granulated sugar with an electric mixer until the mixture becomes pale, thick, and fluffy.
Gently fold the melted chocolate and the rum into the egg mixture until well combined.
Allow the mixture to cool to room temperature.
In a separate bowl, whip the heavy cream until soft peaks form. Gently fold the whipped cream into the chocolate mixture, being careful not to deflate it.
Divide the mousse among ramekins or dessert glasses and refrigerate for at least 2 hours before serving. Enjoy!