Place milk, 1/2 lb butter, both sugars and slat in a large heavy pan. Bring to boil, cook to "softball stage" <this is very hard, 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" between each as they will flatten out. Remember This Candy Is Stirred Constantly While Being Cooked And Beaten Constantly When Removed From The Fire.