1. Combine sugar, soda, salt and buttermilk in a large saucepan.
2. Bring to boiling over high heat, stirring until mixture reaches 210° on a candy thermometer.
3. Add butter and pecans. Cook over medium heat, stirring often, until mixture reaches 230°. (Syrup will form a 2-inch thread when dropped from a spoon.) Remove from heat; add vanilla.
4. Allow to stand until bubbling stops. Beat with wooden spoon until mixture loses its glossiness. Drop in mounds on waxed paper.