1. Separate cold eggs into whites and yolks. Place the whites in a large bowl and yolks in a smaller bowl; allow the whites to come to room temperature before whipping.
2. Whisk the egg yolks until smooth.
3. Melt the chocolate and butter together, either in the microwave or over a double boiler, stirring until smooth. Cool slightly but keep warm (about 35–40°C or 95–104°F).
4. Whip the cream in a chilled bowl until stiff peaks form.
5. Add the caster sugar to the egg whites and beat until firm peaks form.
6. Fold the egg yolks into the whipped cream using approximately 6–8 folds.
7. Gently fold the warm melted chocolate into the cream and yolk mixture with up to 8 folds.
8. Incorporate about one-quarter of the beaten egg whites into the chocolate mixture.
9. Pour the chocolate mixture into the remaining egg whites and gently fold together until just combined.
10. Divide the mousse into 4 small serving glasses or pots.
11. Refrigerate for at least 6 hours, preferably overnight.
12. Garnish with whipped cream, chocolate shavings, raspberries, or mint before serving.