In a pan put 2 c. sugar with canned milk, heat until simmering. In another pan over low heat stir in remaining sugar until melted and clear golden in color. Pour this mixture very slowly into the milk and sugar mixture stirring as you do so.
Cook slowly until 238 degrees or when put in cold water forms a ball. Remove from heat and add vanilla, pecans and butter. Beat unitl mixture begins to thicken and drop by spoonfulls onto waxed paper. Let cool and set up.