2: Pour 60 ml dry white wine and 40 ml white (spirit) vinegar into a saucepan on medium heat. Salt and pepper, then add the chopped shallot. Bring to the boil and leave to reduce gently by 3/4, until very little liquid is left. You can add 2 tablespoons liquid cream at this stage, which is not essential, but - apparently - this is the secret of a successful beurre blanc.
3: Turn down the heat and begin incorporating 250 g butter (very cold) gradually in small pieces, beating continuously.
4: Only add more butter once the previous pieces have melted.
5: Continue until all the butter has been incorporated and you have a smooth sauce. Check seasoning.
6: Beurre blanc can be kept hot for a few minutes at 50°C (122°F), but not too long. Serve in a sauce boat or on the plates.
Remarks
For a smoother sauce, you can finish the sauce by passing it through a fine strainer or sieve, to eliminate the bits of shallot, or use a blender.