8 ounces unsalted butter – cold, cut into 0.5-inch cubes
0.5 tsp fine sea salt
0.125 tsp ground white pepper
Instructions
Finely mince the shallot.
Add it to a small heavy saucepan with the dry white wine and white wine vinegar; bring to a brisk simmer over medium heat and cook until reduced to about 2 tbsp and syrupy, 5–7 minutes.
Lower the heat to very low; whisk in the heavy cream (if using) and let it just bubble for 10–15 seconds, then remove the pan from the heat.
Off the heat, whisk in the unsalted butter a few cubes at a time, letting each addition nearly melt before adding more; as needed, briefly return the pan to low heat to keep the mixture just warm, and continue 3–5 minutes until the sauce is thick, smooth, and glossy.
Season with the fine sea salt and ground white pepper; serve immediately, keeping the sauce warm but below a simmer so the emulsion stays intact.