Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan with non-stick spray or butter.

In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt until well combined.

In another bowl, mix the melted butter, milk, eggs, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay.

In a small bowl, combine the ground cinnamon and brown sugar.

Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture over the batter. Pour the remaining batter on top and finish with the rest of the cinnamon-sugar mixture.

Using a knife or a skewer, gently swirl the batter to create a marbled effect.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

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