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

In a large bowl, whisk together flour, baking powder, salt, and ground cinnamon until well combined.

Cream together butter and granulated sugar until light and fluffy, about 3-4 minutes.

Add eggs one at a time, mixing well after each addition. Stir in milk and vanilla extract.

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

Pour half of the batter into the prepared loaf pan, sprinkle with half of the cinnamon-sugar topping, add remaining batter, and top with remaining topping.

Gently swirl through the batter with a knife to create a marbled effect.

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

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