Preheat your oven to 350°F (175°C) and lightly grease a 9x5-inch loaf pan.
In a small bowl, mix 1/3 cup of sugar with 2 teaspoons of cinnamon, then set it aside.
In a large bowl, combine the flour, the remaining 1 cup of sugar, baking powder, and salt.
In a separate bowl, whisk together the egg, milk, and oil until well blended.
Gradually add the wet ingredients to the dry ingredients and stir until just combined.
Pour half of the batter into the greased loaf pan, spreading it evenly.
Sprinkle half of the cinnamon-sugar mixture over the batter in the pan.
Add the remaining batter on top, spreading it gently over the first layer.
Sprinkle the remaining cinnamon-sugar mixture evenly on top of the batter.
Use a knife to swirl through the layers, creating a marbled effect.
Bake for 45–50 minutes, or until a toothpick inserted into the center of the loaf comes out clean.
Cool the bread in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
Wrap in foil and let sit overnight before slicing for the best flavor and texture.