In a large saucepan, melt butter and then whisk in the sugar, buttermilk, vanilla, and cinnamon. Bring to a boil, then reduce to a simmer and simmer for 2 minutes, whisking constantly.

Add in baking soda and whisk cooking 30 seconds before removing from heat. The mixture will foam up but will go away after a few minutes.

Allow to cool slightly and serve warm. It will thicken as it cools.