In a large saucepan over medium heat, add butter, buttermilk, and sugars. Combine ingredients thoroughly and bring to a boil. Once at a boil, let mixture boil for 1 minute.
Remove from heat and stir in vanilla and baking soda, the mixture will foam up quite a bit. Keep stirring for 1 minute. The foam will reside as the mixture cools. Serve warm or cool.