1In 8-inch skillet, melt 2 tablespoons of the butter over medium-high heat. Add shallots; cook 1 minute, stirring frequently. Add wine; heat to boiling over high heat. Reduce heat to medium-high; cook until mixture coats back of metal spoon, 10 to 12 minutes.
2Stir in broth. Heat to boiling. Reduced heat to medium-high; cook until mixture coats back of metal spoon and is slightly reduced, 8 to 10 minutes longer, stirring constantly.
3With whisk, beat in remaining 4 tablespoons butter, 1 tablespoon at a time. Reduce heat to low. Beat in vinegar, mustard and pepper.