Preheat your oven to 350°F (175°C). Grease and flour a loaf pan (9x5 inches works great).
In a small bowl, mix the brown sugar and cinnamon. Set aside.
In a large mixing bowl, cream together the butter, white sugar, and eggs until light and fluffy. Add the vanilla extract and mix again.
In another bowl, whisk together the flour and baking powder. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture over the batter. Add the remaining batter on top and swirl with a knife or toothpick to create the cinnamon swirl.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the loaf to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely. Slice and enjoy!