Combine. In a large pot, combine butter, buttermilk, corn syrup, sugar, baking soda, and salt. (Do NOT add vanilla at this point.)
Boil. Stirring regularly, bring syrup to a boil (the syrup will get quite foamy). Boil 6-7 minutes, stirring very regularly. The foamy syrup will turn a golden color.
Remove From Heat & Finish. Remove pan from heat. Allow bubbles to settle, then stir in vanilla. Serve warm over pancakes, waffles, french toast, spice cake, ice cream, and more!
Store leftover buttermilk syrup in an airtight container in the refrigerator up to 1 week. Syrup will thicken significantly in the fridge, so bring it to room temperature or warm it up on the stove or in the microwave before using the leftovers.