Place chopped chocolate in a medium heatproof bowl. Heat the cream in a small saucepan over medium heat until it begins to gently simmer. (Do not let it come to a rapid boil—that’s too hot!) Pour over chocolate, then let it sit for 2–3 minutes to gently soften the chocolate.

With a metal spoon or small silicone spatula, slowly stir until completely combined and chocolate has melted. Set aside to slightly cool.

Place egg whites, sugar, cream of tartar, and salt in a heatproof bowl. Set bowl over a saucepan filled with 2 inches of simmering water. Do not let the bowl touch the water. (You can use a double boiler if you have one.) Whisk constantly until sugar is dissolved and mixture has thinned out, about 4 minutes. The mixture will be thick and tacky at first, then thin out and appear frothy on top. To test that it’s ready, you can use your finger or an instant-read thermometer. Lightly and quickly dip your finger (it’s very hot, be careful) and rub the mixture between your thumb and finger. You shouldn’t feel any sugar granules. If using a thermometer, the temperature should read 160°F (71°C).

Remove from heat and pour into a heatproof mixing bowl. (No need to let it cool down before continuing.) Using a handheld or stand mixer fitted with a whisk attachment, whip on high speed until stiff glossy peaks form, about 7 minutes.

Pour the chocolate ganache into the meringue and gently fold together with a silicone spatula until completely combined.

Spoon or pipe the mousse into dessert dishes, ramekins, or jars. Refrigerate for at least 4 hours or up to 48 hours. After about an hour in the fridge, cover the mousse.

Top with whipped cream, chocolate curls, raspberries, and/or fresh mint to serve.