Heat the wine, vinegar and shallot in a pan until bubbling, then reduce the heat slightly and cook until the liquid has reduce to 2 tbsp.
Cut the butter into small cubes, but make sure they stay fridge-cold.
Strain the liquid, return to the pan, then whisk in the cubes of butter one at a time, whisking continually while the mixture emulsifies and thickens.
Remove from the heat once the butter has nearly all been used and continue whisking, then whisk in the final couple of cubes. Taste and season with salt before serving warm.