Combine vinegars with chopped shallot, bay leaf and peppercorns
in saucepan; bring to boil, reduce heat and simmer until liquid is
reduced by half. Strain, reserve liquid, cool.
Put egg-yolks into blender, turn onto slow speed. Gradually add
hot melted butter, no more than a teaspoon at a time to begin with.
When mixture thickens (and this is most important - the vinegar
should not be added before the mixture thickens or the sauce will
become too thin), add 1 teaspoon vinegar mixture; blend a few
seconds then add another teaspoon of vinegar.
Continue adding melted butter gradually until mixture
re-thickens, then add another teaspoon of vinegar.
Continue in this way, blending in butter, then vinegar, until
all butter and vinegar have been used.
