In a medium saucepan over medium heat, add the butter, sugar, buttermilk, and corn syrup. Stir occasionally while the mixture comes to a rapid boil. Once a boil is reached, whisk constantly for 3 minutes. Remove the mixture from the heat and add in the vanilla and baking soda and whisk constantly until incorporated (the mixture will bubble up a bit when the baking soda is added).
Serve warm. Place leftover Buttermilk Syrup in a mason jar in the refrigerator.