1. Melt the chocolate and cocoa butter in a double boiler, or heatproof bowl placed over a small saucepan filled with 1 cup of water, over medium heat.
2. Let the melted chocolate cool to room temperature. In the meantime, separate the egg whites from the egg yolks. Using an electric mixer, beat the egg whites and cream of tartar. As the egg whites thicken, slowly add the powdered Erythritol. Beat until they create stiff peaks.
3. In another bowl, whisk the cream until soft peaks form when the whisk is removed.
4. In a third bowl, mix the egg yolks with the vanilla extract.
5. When the chocolate has cooled down to room temperature, using a rubber spatula, add about a third of the fluffy egg whites and mix with the chocolate. Then add the remaining egg whites and gently fold in without deflating them.
6. Slowly stir in the egg yolk and vanilla mixture. Finally, fold in the whipped cream using the spatula, creating fluffy chocolate mousse.
7. Transfer the chocolate mousse into a deep baking dish (I used a 9 x 9 inch baking tray lined with heavy-duty parchment paper). Freeze for at least 4-6 hours, or until set.

Tip: You can use single-serving containers for portion control. If the ice cream is too hard, leave it at room temperature for 15-20 minutes before serving.