Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan.

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

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

Add the wet ingredients to the dry ingredients and stir until just combined.

Pour half the batter into the loaf pan.

(Optional) Mix the extra sugar and cinnamon and sprinkle half over the batter.

Add the remaining batter and top with the rest of the cinnamon sugar.

Swirl with a knife for a marbled effect if desired.

Bake for 45–55 minutes or until a toothpick inserted comes out clean.

Let cool in pan for 10 minutes, then transfer to a wire rack to cool completely.