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

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

Cream together softened butter and granulated sugar until light and fluffy, about 3 to 5 minutes.

Add eggs one at a time, mixing well after each addition, then stir in milk and vanilla until just combined.

Gradually add the dry ingredients to the wet mixture, mixing gently until just combined.

Pour half of the batter into the prepared loaf pan, sprinkle with half of the cinnamon-sugar mixture.

Pour the remaining batter over the cinnamon-sugar layer and top with remaining cinnamon-sugar mixture, swirling slightly into the batter.

Bake for 50 to 60 minutes or until a toothpick inserted in the center comes out clean.

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