Combine buttermilk, sugar, butter, corn syrup, and baking soda in a LARGE pot. Like one you’d make soup in. Yes, you’ll have way more pot than ingredients, but this will boil all over your newly-cleaned stove if you put it in a smaller saucepan.
Bring ingredients to a boil and reduce heat to low (as long as it’s still bubbling, you’re okay). Cook, stirring very frequently, for 8-9 minutes. You’re basically making candy here and candy-making requires constant vigilance.
When it’s done, it should take on this luscious golden-brown color. Remove from heat and add vanilla.