Beat eggs for 10 minutes, till very pale and fluffy. Beat the whipping cream in a separate bowl to stiff peaks.
Place chocolate in a heatproof bowl. Place the bowl over a pan of simmering water, ensuring that the bowl does not touch the water. Melt the chocolate, then add the butter. Once entirely melted, remove from heat.
Place another heatproof bowl over the pan of simmering water. Add the beaten eggs. Whisk the eggs constantly, till thickened and warm.
Combine the egg and chocolate mixtures. Fold in the whipped cream, and gently fold together till well combined. Enjoy straightaway.