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 cinnamon.

In a separate bowl, cream together butter and sugar until light and fluffy.

Add eggs one at a time, mixing well. Pour in milk and vanilla extract, beat until smooth.

Gradually incorporate the dry mixture into the wet ingredients, stirring gently.

Pour half of the batter into the loaf pan, sprinkle with half the cinnamon sugar, then pour remaining batter and top with remaining cinnamon sugar.

Gently swirl the batter with a knife and bake for 50-60 minutes until a toothpick comes out clean.

Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.