In a large saucepot, melt the butter and add the oats. Stir for 2 minutes to toast. Add the boiling water and reduce heat to a simmer. Keep at a low simmer for 25 minutes, without stirring.
Combine the milk and half of the buttermilk with the oatmeal. Stir gently to combine and cook for an additional 10 minutes. Spoon into a serving bowl and top with remaining buttermilk, brown sugar, and cinnamon.