Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x5-inch loaf pan.

Mix together 1/3 cup sugar and 2 teaspoons cinnamon in a small bowl; set aside.

Combine flour, remaining 1 cup sugar, baking powder, and salt together in a large bowl. Combine egg, milk, and oil in a separate bowl; add to flour mixture. Stir until just moistened.

Pour 1/2 of the batter into the prepared pan. Sprinkle with 1/2 of the reserved cinnamon-sugar mixture. Repeat with remaining batter and cinnamon-sugar mixture. Draw a knife through batter to marble.

Bake in the preheated oven until a toothpick inserted into the center of the loaf comes out clean, 45 to 50 minutes.

Let cool in the pan for 10 minutes before removing to a wire rack to cool completely. Wrap in foil and let sit overnight before slicing.

Serve and enjoy!