In a 3 to 4-quart saucepan (make sure it's large enough, this will bubble up!) over medium heat, combine the butter, sugar, buttermilk, corn syrup, and salt. When butter is mostly melted, turn heat up just a bit to medium-high and bring mixture to a boil. It will foam up and rise in volume.

Reduce heat to medium-low. The mixture should still be bubbling, but the volume will settle down. Simmer for 8-10 minutes, stirring frequently and scraping the bottom of the pan. The mixture will still be a pale yellow color at this point. (If you want a lighter caramel, simmer for 5-7 minutes. For a darker caramel with deeper flavor, simmer for up to 10 minutes.)

Remove pan from heat. Sprinkle the baking soda evenly over the top and stir to combine. The syrup will bubble up and look very foamy. Then stir in the vanilla.

Let sit for 20 minutes, stirring every 5 minutes or so. The bubbles will settle and the caramel syrup will thicken and darken as it cools.

Serve caramel syrup with apple oat pancakes, pancakes, waffles, French toast, and even ice cream!