Add vinegar, white wine, and chopped shallots to a saucepan. Place it over medium heat. Let it simmer gently. Cook until the liquid reduces to about two tablespoons. This step builds strong flavor.

Turn the heat to low. This step is very important. High heat can break the sauce. Keep the pan warm but not too hot. You should see only gentle steam.

Add one cube of cold butter into the pan. Whisk constantly until it melts fully. Keep adding butter one piece at a time. Do not rush. Slow mixing keeps the sauce smooth.

Continue whisking while adding butter cubes. The sauce will become creamy and thick. Make sure each cube melts before adding the next one. This step takes patience but gives perfect texture.

Once all butter is added, mix in salt and black pepper. Add fresh lemon juice. Stir well to balance the flavors. Taste the sauce and adjust seasoning if needed.

If you want a smooth finish, strain the sauce. Remove shallots using a fine strainer. This step is optional but gives a restaurant-style look.

Serve the sauce warm. Do not let it sit too long. Beurre blanc tastes best when fresh. Pour it over fish, chicken or vegetables for a rich finish.