Combine the butter, sugar, buttermilk, and Karo syrup in a pot and bring to a boil for 2-3 minutes, stirring frequently.
Add the baking soda and vanilla and stir till combined. The color of the syrup will turn to a light golden brown. Remove from the heat and continue to stir till the syrup thickens. Let it cool and serve immediately, or store in fridge in an air tight container. (you can warm it up in the microwave)
*Make sure to use a pot with higher sides because the syrup will foam up to double it's size.