Whip cream to soft peaks and chill.
Add chocolate and butter to a heatproof bowl. Place the bowl over a pan of gently simmering water and melt , stirring frequently until smooth.
Remove the chocolate mixture from the heat and let it cool for 10 minutes or until warm to touch.
Separate the eggs. Add the yolks to the cooled chocolate.
Add egg whites to a grease free bowl. Whip the egg whites until foamy, then add in cream of tartar. Sprinkle in the sugar and whip until stiff peaks form.
Fold in half of the whipped cream into the chocolate, then gently fold half the whipped egg whites with a spatula. Fold in remaining whipped cream and finally fold in remaining egg whites.
Spoon into serving glasses or bowls. Chill in the fridge for 8 hours.