Beurre blanc
Julia Child
1/4 cup white wine vinegar     Boil until reduced to 1 1/2 tablespoons. Remove from heat.   Set over low heat and continue beating in a piece of butter at a time just as the previous piece is almost fully incorporated. The sauce should have the consistency of a light hollandaise.
1/4 cup lemon juice  
1 tablespoon shallots finely chopped
1/4 teaspoon salt  
1/8 teaspoon pepper  
2 oz unsalted butter chill   Immediately beat in half the butter, until it forms a creamy paste. Beat in the rest.
10 oz unsalted butter  
salt and pepper     Remove from heat, season to taste
lemon juice