2 tablespoons unsalted butter, plus 2 sticks, cut into small cubes and very cold
1 shallot, sliced
1 1/2 cups dry white wine
1/2 cup seasoned rice vinegar
2 teaspoons whole peppercorns
1 tablespoon soy sauce
In a medium saucepan, melt 2 tablespoons butter over medium-low heat. Add the shallots and cook until softened, 3 to 4 minutes. Add the wine, vinegar, peppercorns, and soy sauce, bring to a simmer, and cook until the liquid is syrupy and reduced to about 1/4 cup, 20 to 25 minutes.
Strain the liquid through a fine mesh strainer, discard the shallots and return the liquid to the pan. Return to medium-low heat and bring to a low simmer, allowing the liquid to reduce a bit more. While whisking constantly, slowly add about 3/4 of the cold butter cubes, a couple at a time, until emulsified. Turn off the heat and continue whisking while slowly adding the remaining butter cubes; whisk very well until thick and creamy.
Use immediately or store in a warmed thermos bottle or covered in a glass bowl set over warm water until ready to serve. Refrigerate leftovers.