In a small saucepan, combine vinegar and white wine. Bring to a low boil over medium heat, then add shallot and a pinch of white pepper. Reduce to a syrupy glaze, 6-8 minutes.
Remove the pan from heat and whisk in two lumps of butter, then two more.
Set pan over lowest possible heat and continue to whisk in more butter as soon as the last piece has been absorbed. This should take around 10-15 minutes.
If the sauce is melting too much, remove it from the heat completely. It should be smooth like mayonnaise, and the butter should not be fully melted but creamy instead.
Add salt to taste and serve. Letting it rest a moment before serving will help the sauce thicken further and prevent melting when added to hot food.
Notes
Makes about 2 cups.
Make sure to use high quality butter, such as Kerrygold or Plugra.