In a large 3 1/2 quart-size saucepan, combine sugar, baking soda, buttermilk, and salt. Cook over high heat for about 5 minutes or until the mixture reaches 210 degrees on a candy thermometer. Stir often scraping the bottom and sides. The mixture will foam up.
Add butter, pecans, and vanilla.
Over medium heat, continue cooking.
Stir constantly scraping bottom and sides until the mixture reaches softball stage (235 degrees on a candy thermometer or spinning a thread).
Remove mixture from heat.
Cool slightly (about 2 minutes). Beat with spoon until thick and creamy.
Drop from a tablespoon onto sheet of alumium foil or wax paper. Let cool. Store in an airtight container.