Break the chocolate into a large heatproof bowl and set it over a pan of gently simmering water and stir until melted. Set aside.
Place the egg yolks and sugar in a medium bowl and place it over the hot water in the pan. Use an electric whisk to beat the mixture until it is pale and fluffy. Remove the bowl from the heat and continue to whisk until it is really thick.
Add the chocolate to the egg yolk mixture with 1 tbsp hot water and fold in until smooth. Now add the Quark and gently fold together until the mixture is evenly mixed.
Divide between four glasses and chill for 1 hour. Optional: decorate with a little more of the Quark and some fresh raspberries.