Place chopped chocolate in a medium heatproof bowl.
Heat heavy cream in a small saucepan over medium heat until it gently simmers but does not boil.
Pour hot cream over the chopped chocolate and let rest for 2–3 minutes to soften.
Stir gently with a spoon or spatula until chocolate is fully melted and combined. Set aside to cool slightly.
Combine egg whites, sugar, cream of tartar, and salt in a heatproof bowl over a saucepan with simmering water without touching the water.
Whisk constantly until sugar dissolves and mixture thins, about 4 minutes, reaching 160°F or no sugar grain feel.
Remove from heat and transfer to a mixing bowl. Whip with a mixer on high until stiff, glossy peaks form, about 7 minutes.
Gently fold the chocolate mixture into the whipped egg whites until fully combined.
Spoon or pipe the mousse into serving dishes and refrigerate for at least 4 hours, covering after 1 hour.
Serve chilled, optionally topped with whipped cream, chocolate curls, raspberries, or fresh mint.