Preheat your oven to 350°F (175°C).
In a mixing bowl, combine flour, sugar, cinnamon, baking powder, and salt.
In another bowl, whisk together milk, oil, and egg until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
In a small bowl, mix together the brown sugar and cinnamon for the filling.
Pour half of the batter into a greased loaf pan.
Sprinkle half of the cinnamon-sugar mixture over the batter.
Add the remaining batter on top and finish with the rest of the cinnamon-sugar mixture.
Use a knife to swirl the batter gently to create a marbled effect.
Bake in the preheated oven for 40-45 minutes or until a toothpick comes out clean.