salt and white pepper (so that it doesn't color your sauce)
Cook the shallots in the vinegar and wine in a small heavy saucepan on low heat until there is only two tablespoons of liquid.
Stir in the heavy cream, bring to a low boil on higher heat, and reduce once again to two tablespoons of liquid.
On low heat, add the butter, piece by piece, to the reduced wine whisking continually. Fully incorporate each piece of butter before continuing. From time to time, remove pan from heat to prevent the sauce from over-heating (you don't want it to boil, but you want it stay warm.)