White butter sauce

A recipe by M. Gueho, L'Atlantide restaurant,1 Michelin star

White butter sauce is a typical recipe of the Nantes region. It is served with fish and is the ultimate dish for family celebrations!
    
Ingredients (for 6 to 8 people):
●    10 shallots
●    1dl dark wine vinegar
●    1dl Muscadet
●    2 soup spoons double cream
●    500g lightly salted butter
●    Ground pepper

Preparation:
Cut the shallots very finely with a knife.
Put the shallots, vinegar, Muscadet and ground pepper together in a pan or a heavy saucepan.
Bring to the boil, lower the heat and leave to evaporate slowly.
Then add the double cream.
Bring to the boil.
Mix in the butter, cold and cut into cubes, using a whisk.
The butter is to be added quickly and whisked energetically. Keep it on the heat, over a low flame, using a water bath or on the edge of the ring if necessary.
Do not sieve the mixture. White butter sauce is served with shallots and is the perfect accompaniment for pollock, zander, shad or pike cooked in a court-bouillon.

Note: 
In traditional Nantes cuisine white butter sauce doesn’t include cream, but adding cream is a trick for making the sauce more stable.