Place milk, half 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 half 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.