Melt butter over medium heat, bring to the boil and skim off the foam with a ladle. Remove from the heat and let it cool down a little. Wash ##chervil## and shake dry.
Pluck leaves from the stems, except for a few for garnishing, and chop finely. Beat egg yolks and white wine over a hot water bath until frothy. Gradually fold in half of the liquid butter.
Then slowly stir in yoghurt and the remaining butter. Sauce should have a thick, slightly frothy consistency. Add chopped chervil and season to taste with a little sugar, salt and ##pepper##.
Arrange the sauce and garnish with chervil.