Preheat your oven to 350°F (175°C) and grease a 9×5 inch loaf pan.

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

In a separate bowl, cream the softened butter and granulated sugar until fluffy, about 3-4 minutes.

Add eggs one at a time, then mix in milk and vanilla until combined.

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

Pour half of the batter into the loaf pan, then sprinkle half of the cinnamon-sugar topping on top. Layer the remaining batter and finish with the rest of the topping.

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

Bake for 50-60 minutes or until golden brown and a toothpick comes out clean.

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