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

Whisk together the dry ingredients in a large bowl: flour, granulated sugar, baking powder, salt, and cinnamon.

Combine the wet ingredients in a separate bowl: mix the milk, eggs, melted butter, and vanilla extract until smooth.

Pour the wet mixture into the dry ingredients and stir until just combined.

Make the swirl filling by combining brown sugar and cinnamon in a small bowl.

Layer the batter: pour half into the prepared pan, sprinkle half of the cinnamon-sugar mixture over the top, add the remaining batter, and top with the rest of the cinnamon-sugar mixture.

Swirl through the batter gently using a knife or skewer.

Bake for 50-60 minutes or until a toothpick comes out clean. Cool in the pan for 10 minutes before transferring to a wire rack.