Preheat your oven to 350°F (175°C). Grease a loaf pan.
In a mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, mix the milk, egg, vanilla extract, and melted butter.
Combine the wet and dry ingredients until just mixed.
Pour half of the batter into the loaf pan. Sprinkle half of the cinnamon over the batter. Pour the remaining batter over the cinnamon layer, then sprinkle the remaining cinnamon on top.
Use a knife to swirl the cinnamon into the batter.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.