In a heavy 2 quart saucepan, combine sugar, butter, buttermilk, baking soda, and corn syrup over high heat.
Cook, stirring occasionally, until mixture reaches the hard ball stage (268 degrees on a candy thermometer).
Remove from heat.
Stir in vanilla and pecans.
Place pan in a large bowl of ice and beat with a wooden spoon just until mixture begins to thicken.
Quickly spread in an 8 inch square pan.
Allow to cool thoroughly before cutting into squares.