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

In a large bowl, whisk together 2 cups of all-purpose flour, 2 teaspoons of baking powder, ½ teaspoon of salt, and 2 teaspoons of ground cinnamon.

Cream together ½ cup of unsalted butter and 1 cup of granulated sugar until light and fluffy.

Add 2 large eggs one at a time, beating well after each addition, then stir in ½ cup of milk and 1 teaspoon of vanilla extract.

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

Pour half of the batter into the prepared loaf pan and sprinkle half of the cinnamon sugar mixture over it.

Add the remaining batter and cover it with the rest of the cinnamon sugar mixture.

Using a knife or a skewer, gently swirl the batter in the pan to create a marble effect.

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

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