In a saucepan, combine the shallots and Champagne. Scrape the vanilla seeds from the split pod into the pan and add the vanilla pod halves. Bring to a boil and cook until almost all liquid evaporates, about 6 minutes.
Add the heavy cream, bring to a boil and simmer until cream thickens and coats the back of a spoon, and is reduced by nearly 50 percent in volume, about 3 minutes.
Whisk in the butter until smooth and strain the sauce through a fine mesh strainer, pressing on the solids to extract as much liquid as possible. Season, to taste, with salt, pepper, and vinegar and serve immediately over fish or shellfish.
Note: If sauce is made in advance, keep warm in a bain marie over barely simmering water.
![]("/food/images/spacers/spacer.gif"){kind=spacer}
