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

In a large bowl, whisk together flour, baking powder, salt, and ground cinnamon.

Cream together butter and sugar until light and fluffy.

Beat in eggs, then stir in milk and vanilla extract until smooth.

Gradually add dry ingredients to wet mixture, stirring until just combined.

Pour half the batter into the pan, sprinkle with half the cinnamon-sugar mixture, top with remaining batter and sprinkle with the rest.

Swirl the batter using a knife or skewer for a marbled effect.

Bake for 50-60 minutes, until a toothpick comes out clean.

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