Heat 4 Tablespoons Butter In A Large Heavy Skillet Over Medium Heat. Cook Chicken Breasts Until Golden Brown, Turning Once. Pour In 1/2 Cup Vermouth. Cover, And Simmer Until No Longer Pink, And Juices Run Clear, About 15 Minutes. In A Saucepan, Heat 2 Tablespoons Butter Over Medium Heat. Saute Onion Until Soft And Translucent, But Not Brown. Stir In 1/4 Cup Vermouth And Sour Cream; Remove From Heat. When Chicken Is Done, Pour Sauce Over. Season With Salt And Pepper. Heat Only Long Enough To Warm The Sauce, But Do Not Boil.