Place the finely chopped semi-sweet chocolate in a heatproof bowl. Heat the heavy cream until it is just simmering, then pour it over the chocolate. Let the mixture sit undisturbed for 2 to 3 minutes, then stir gently until smooth and fully combined. Allow the ganache to cool slightly before proceeding.

In a heatproof bowl, combine the egg whites, granulated sugar, cream of tartar, and a pinch of salt. Set the bowl over a saucepan of simmering water, ensuring that the bottom of the bowl does not touch the water. Whisk continuously for about 4 minutes until the mixture is warm and the sugar has dissolved.

Remove the bowl from the heat and whip the warmed egg white mixture on high speed until stiff peaks form. Be careful not to overbeat.

Fold a single large scoop of the meringue into the ganache to lighten it. Then gently fold the lightened ganache back into the remaining meringue until uniform in color and texture.

Spoon or pipe the mousse into serving dishes or ramekins. Smooth tops if desired. Refrigerate for at least 4 hours to allow the mousse to set and flavors to meld.

Add any optional toppings you prefer just before serving and enjoy!