In a large saucepan over medium-high heat, melt the butter. (Note: Select a saucepan larger than you anticipate, as the mixture expands considerably when baking soda is added.)

Whisk in the sugar, buttermilk, vanilla, and cinnamon into the melted butter, and bring the mixture to a boil.

Once boiling, reduce the heat and simmer for 2 minutes, whisking constantly.

Whisk in the baking soda. The mixture will foam significantly. Continue cooking for an additional 30 seconds before removing from the heat.

Allow the syrup to cool; it can be served warm or cold. The syrup will thicken as it cools.

Transfer to a jar for storage in the refrigerator.