Preheat oven to 350°F (175°C). Grease and flour a 9×5 inch loaf pan.
In a large bowl, whisk together the flour, baking powder, and salt.
In another bowl, mix the melted butter, 1 cup of sugar, eggs, milk, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Pour half of the batter into the prepared loaf pan.
In a small bowl, mix 1/3 cup of sugar with the ground cinnamon.
Sprinkle half of the cinnamon-sugar mixture over the batter in the pan.
Pour the remaining batter over the cinnamon-sugar layer, then sprinkle the rest of the cinnamon-sugar mixture on top.
Use a knife to swirl the batter to create a marbled effect.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
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