Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan with butter or cooking spray.

In a small bowl, combine granulated sugar with ground cinnamon.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt.

In a mixing bowl, beat the unsalted butter and granulated sugar together until fluffy.

Add the egg, milk, vanilla extract, and vegetable oil into the butter-sugar mixture and mix until smooth.

Gradually add the dry mixture into the wet ingredients, stirring until just combined.

Pour half of the bread batter into the prepared loaf pan, sprinkle half of the cinnamon sugar mixture over it, then add the rest of the batter and top with remaining cinnamon sugar.

Gently swirl the cinnamon sugar mixture into the batter using a toothpick or knife.

Bake for 50-60 minutes, or until a toothpick inserted comes out clean.

Let the bread cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.