1. Combine wine and vinegar in a small saucepan. Bring to a simmer over medium-high heat. Reduce heat to medium, and simmer until liquid is reduced by three-quarters, about 12 minutes.

2. Set a heat-proof bowl, or the top of a double boiler, over a pan of simmering water. Transfer the liquid to the bowl. Lightly beat egg yolks, and whisk them into the wine reduction, whisking constantly until the mixture is thick enough to form ribbons, about 3 minutes.

3. Remove from heat, and slowly add olive oil, pouring in a steady stream and whisking constantly until all the olive oil is incorporated. Whisk in mustard, and season with salt and pepper. Serve immediately.