Combine white wine vinegar, white wine, diced shallots, black peppercorns, and 2 tablespoons of tarragon in a small saucepan.

Bring the mixture to a boil over high heat.

Cook until the liquid is reduced to approximately 3 tablespoons (about 8 minutes).

Strain the reduced liquid into a small bowl, pressing against the solids to extract maximum flavor.

Allow the strained liquid to cool to room temperature.

Discard the strained solids.

Place the softened unsalted butter in a medium mixing bowl.

Add the cooled liquid reduction to the butter.

Incorporate the remaining 2 tablespoons of fresh tarragon, kosher salt, coarsely ground black pepper, and fresh parsley into the butter mixture.

Mix all ingredients thoroughly until well combined.

Cover the bowl tightly.

Refrigerate for at least 1 hour, or up to 2 days, to allow the flavors to meld together.

Remove from the refrigerator 30 minutes before serving to soften slightly.