Place milk, 1/2 lb butter, both sugars and salt in a large heavy pan. Bring to boil, cook to "softball stage" (You MUST use a candy thermometer.) Remove from heat and add 1/2 lb butter. Beat constantly. When you feel it begin to thicken, add pecans and vanilla. When thick QUICKLY drop by teaspoon on greased surface or foil. Leave 2-3 inches between each as they will flatten out. REMEMBER THIS CANDY IS STIRRED CONSTANTLY WHILE BEING COOKED AND BEATEN CONSTANTLY WHEN REMOVED FROM THE FIRE.