Preheat the oven to 350°F and spray a loaf pan with cooking spray or grease it with butter. In a bowl whisk the brown sugar with the cinnamon together until combined.
Combine the flour, baking powder, baking soda and salt together in a bowl. Set aside. In another bowl cream the butter with the granulated sugar until light in color and well combined. Add the eggs in one at a time and mix until light and fluffy.
Add the buttermilk, vanilla extract and flour mixture to the egg mixture and mix until well incorporated and smooth. Note: the batter is a bit thicker than a regular cake batter. If your batter is too thick, add a bit more buttermilk, or if it's too thin, add a bit more flour.
Pour ⅓ of the batter in the prepared bread pan and sprinkle with ⅓ of the brown sugar mixture, repeat with remaining batter and sprinkle remaining brown sugar mixture over the top. You should end up with 3 layers of batter and 3 layers of cinnamon sugar.
Transfer the loaf pan to the oven and bake for 50 minutes to 1 hour or until a toothpick comes out clean inserted in the center of the loaf. Cool in the loaf pan for 5 minutes, then transfer to a wire rack to finish cooling.
