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

Whisk together the flour, baking powder, salt, and ground cinnamon in a large bowl and set aside.

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

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

Fold the dry ingredients into the wet mixture until just combined; do not overmix.

Pour half of the batter into the loaf pan, sprinkle half of the cinnamon-sugar mixture, then add the rest of the batter and the remaining topping.

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

Bake for 50-60 minutes, checking for doneness with a toothpick.

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

Slice and serve warm with butter or enjoy as is.