Preheat Oven: Set your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan.

Prepare the Batter:

In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon.

In another bowl, combine the milk, egg, oil, and vanilla extract.

Pour the wet ingredients into the dry ingredients and stir until just combined; avoid overmixing.

Create the Cinnamon Swirl:

In a small bowl, mix the sugar and cinnamon for the swirl.

Assemble the Bread:

Pour half of the batter into the prepared loaf pan.

Sprinkle the cinnamon-sugar mixture evenly over the batter.

Pour the remaining batter over the cinnamon-sugar layer.

Use a knife to gently swirl through the batter to create a marbled effect.

Bake:

Place the loaf pan in the preheated oven and bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and let the bread cool in the pan for about 10 minutes.

Transfer the bread to a wire rack to cool completely before slicing.