Preheat oven to 350°F (175°C) and grease and flour a 9x5-inch loaf pan.
In a small bowl, combine brown sugar and cinnamon. Set aside.
In a medium bowl, combine flour, baking powder, salt, white sugar, and brown sugar.
In a large bowl, combine milk, egg, vegetable oil, and vanilla extract. Whisk until smooth, then add the flour mixture to the bowl and stir until just moistened.
Pour half the batter into the prepared pan. Sprinkle half of the cinnamon sugar mixture on top. Pour the remaining batter on top and sprinkle with the cinnamon sugar mixture. Use a knife and draw it through the batter to marble it.
Bake for 50 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the pan for 10 minutes before transferring to a wire rack to cool completely.