In a large skillet cook mushrooms in 1 tablespoon the hot butter over medium-high heat about 5 minutes or until tender. Remove mushrooms from skillet.
Reduce heat to medium. Add the remaining 2 tablespoons butter and the chicken breast halves to skillet. Cook for 8 to 12 minutes or until no longer pink (170 degrees F), turning once. Remove chicken from skillet and keep warm.
Remove skillet from heat; add vinegar, stirring to loosen browned bits in bottom of skillet. Return skillet to heat. Stir in cream, capers, and pepper. Bring to boiling. Boil gently, uncovered, for 2 to 3 minutes or until sauce is slightly thickened. Return mushrooms to skillet; heat through. Top chicken with mushroom sauce. Makes 6 servings.