In a 2- or 3-quart saucepan, add the sugar, buttermilk and butter (see note). This syrup will foam and expand, so don't use a smaller pan.
Over medium-low heat, stir until the butter is fully melted. Increase the heat to medium and bring to a boil.
Reduce the heat to medium-low and simmer for 5 minutes. You want the heat on the lowest setting that still maintains a simmer. The longer the syrup simmers the thicker and darker it will be. If it cooks too long or at too high of heat, it is more likely to crystallize as it cools (and be too thick to use as syrup).
Off the heat, add the vanilla and baking soda. The syrup will foam and expand. Mix until the ingredients are well-combined. Serve warm.
The leftover syrup can be refrigerated and warmed up before serving. Sometimes a layer of foam develops on the syrup. It doesn't affect the taste; however, you can stir to recombine the syrup or skim off the foam, if you prefer.
Notes
Baking Soda: adding the baking soda at the beginning to simmer with the buttermilk, sugar and butter produces a darker, more caramelly syrup. I don't always make it this way because I also like the lighter, less intense syrup, but it's a delicious option for deeper caramel flavor and color. Larger Batches: add 1-2 minutes to the simmering time for double or triple batches (also don't forget to use a large pot - I use a 6-quart pot for large batches).