Measure sugar and corn syrup into a large saucepan. Stir baking soda and salt into the buttermilk; add to the sugar. Bring mixture to a boil over medium heat, stirring until sugar is dissolved. Continue cooking to soft ball stage* (236° ). Remove from heat and add butter and vanilla. Beat until creamy. Stir in pecans and quickly drop from spoon onto wax paper.
*To Test for Soft Ball Stage
A small amount of syrup dropped into chilled water forms a ball, but is soft enough to flatten when picked up with fingers (234° to 240°).