Preheat your oven to 350°F. Grease a 9x5 inch loaf pan with butter or vegetable oil and line with parchment paper (leaving some hanging up the sides to make it easier to lift the cake out). Set aside.
Make the cinnamon swirl: In a medium sized bowl, combine your brown sugar, cinnamon, and melted butter or vegetable oil. Mix until you get a moist and crumbly mixture. Set aside.
Make the sweet quick bread batter: In a large bowl, combine flour, baking powder, baking soda, and salt. Whisk to combine. Set aside.
In a separate medium bowl, combine eggs, vegetable oil, sugar, buttermilk, and vanilla extract. Whisk to break up the eggs and combine everything. Pour your wet ingredients into the dry ingredients. Fold the ingredients together until just combined. Take care not to over mix!
Assemble: Pour about 2/3 of your quick bread batter into your prepared loaf pan, and smoothen out into an even layer. Sprinkle the cinnamon sugar mixture on top. Using a wooden skewer or knife, swirl the cinnamon sugar into the batter by running the utensil through the batter. Top with the remaining quick bread batter.
Bake your cinnamon swirl quick bread in the center of your preheated 350°F for 50 to 60 minutes, or until a skewer inserted into the center comes out clean of batter (you may see some of the cinnamon swirl on the skewer). If you find your cake is browning too quickly, loosely tent with aluminum foil and continue baking until cooked through. Cool for 10 minutes in the pan, transfer to a cooling rack and cool completely before slicing.