Oil a jelly-roll pan or an 8 X 20 cm pan. Combine the sugar, milk and salt in a heavy 3 litre pot, stirring to mix well. Place over medium heat and bring to a boil, stirring constantly, until the sugar dissolves. Cove and let boil for 2 to 3 minutes. Uncover and wash down the sides of the pot with a pastry brush dipped in cold water. Continue to boil, over medium heat, to the firm ball stage (240-250 degrees F.), stirring only if it starts to burn. Remove from the heat and immediately place the pot in a larger pot filled with cold water; this will stop the cooking process and bring the temperature down. Drop in the butter and let cool slightly, without stirring. beat until it starts to thicken, add the vanilla and the pecans and continue to beat until the candy loses some of its gloss. Spread evenly in the pan and mark into squares. When firm, cut into pieces and store in an airtight container. From Fannie Farmer's New Cookbook