Mix together the cinnamon sugar ingredients and set aside.
For the streusel, mix together flour, brown sugar, and cinnamon well. Add in the cold butter and use a pastry cutter (or food processor) to cut the butter into the mixture until it is crumby but holds together when pinched.
Preheat oven to 350°F (175°C).
In a bowl, cream together the melted butter and sugars well, then add in the egg and vanilla.
In another bowl, whisk together the flour, baking powder, baking soda, and salt.
Add about a third of the flour mixture into the butter/sugar mixture and gently fold.
Alternate by adding about half the buttermilk and fold. Keep alternating between adding the flour and buttermilk.
In a 9x5 loaf pan, pour half the batter, then layer on the cinnamon sugar mixture, followed by the other half of the batter.
Top with the streusel and bake for 50-60 minutes. A toothpick should come out clean from the batter, though you may see cinnamon sugar on the toothpick.