Step 1

Combine the shallot, champagne and vinegar in a small saucepan over medium heat. Once the mixture starts to bubble, cook for 5 to 8 minutes, stirring occasionally, until it has reduced by about a third. Add the cream; cook for about 8 minutes, stirring occasionally, until the mixture has thickened slightly -- enough to coat the back of a spoon. Remove the saucepan from the heat, but keep the burner on, reducing it to low heat.

Step 2

Whisk in the butter a few pieces at a time; return the saucepan to a burner (low heat) if the butter is not melting easily. Season lightly with salt and pepper.

Step 3

Strain through a fine-mesh strainer into a heatproof bowl, if using right away, discarding any solids; or into such a bowl suspended over a pan with a few inches of water (over medium-low heat) to keep the sauce warm (for up to 30 minutes) until ready to serve.