In a large, heavy saucepan with high sides, bring buttermilk, sugars, and butter to a boil over medium heat.
¾ cup buttermilk, ¾ cup brown sugar, ¼ cup granulated sugar, ¼ cup salted butter
Remove from heat and whisk in baking soda, vanilla, and a pinch of salt. It will bubble up a lot, which is why you want to use a large saucepan with high sides.
½ teaspoon baking soda, 1 teaspoon vanilla, Pinch of salt