In a small bowl whisk together the
egg yolks. Whisk 1 tablespoon of the
vinegar mixture into the eggs. Pour the
egg mixture into the saucepan with the
vinegar mixture and whisk until smooth.
Place the pan over low heat and whisk the mixture constantly until it thickens to the consistency of
mustard. Then, immediately start drizzling the melted
butter in a thin stream into the pan, whisking constantly, until the mixture reaches the consistency of thin
mayonnaise. If at any time the
sauce looks like it is separating, remove the pan from the heat and continue whisking to
cool it down. Set the
sauce aside at room temperature.