Lemon Tarragon Beurre Blanc

In a small saucepan boil shallot and lemon juice, tarragon, and vinegar until liquid is reduced to about 1 tablespoon. Reduce heat to low and whisk in butter, 1 piece at a time, adding each new piece of butter only after the previous one has combined into the sauce. Season with salt and pepper to taste and if not using immediately, keep warm in a metal bowl set over a saucepan of hot water.