1. Separate eggs, allowing the whites to reach room temperature for better whipping.
2. Melt chocolate and butter gently in a microwave or using a bain-marie; stir until smooth. Cool slightly.
3. Whisk egg yolks until uniform; optionally, beat with some sugar until thick and pale.
4. Whip 1/2 cup heavy cream until stiff peaks form; avoid overwhipping.
5. Beat egg whites with remaining sugar until firm peaks form; add cream of tartar if desired.
6. Fold egg yolks into whipped cream with gentle motions to keep airiness.
7. Ensure chocolate is warm (95°F/35°C) and fold into cream and yolk mixture.
8. Lighten with a quarter of whipped egg whites, folding about ten times.
9. Gently fold in remaining egg whites until no white streaks remain.
10. Spoon or pipe mousse into serving dishes; refrigerate at least 2 hours to set.
11. Before serving, whip additional cream and garnish mousse with chocolate shavings, raspberries, or mint.