Bring the butter to a rolling boil in a sauce pan, but do not let it brown.

Place remaining ingredients into blender and pulse until yolks are smooth.

With machine running, slowly drizzle (a few drops at first) in the melted butter. Continue to add butter in very slowly - too fast and it won't get emulsified. Stop pouring butter in periodically to let machine incorporate, then continue to slowly drizzle in again. Should take about 5 minutes of blending.