Preheat your oven to 350°F (175°C) and grease a standard loaf pan.

In a large mixing bowl, combine flour, sugar, baking powder, and salt. Whisk until well blended.

In a separate bowl, whisk together milk, melted butter, egg, and vanilla extract until fully combined.

Gradually mix the wet ingredients into the dry ingredients, stirring until just combined.

In a small bowl, mix brown sugar, melted butter, and ground cinnamon until it forms a paste.

Pour half of the batter into the greased loaf pan, sprinkle half of the cinnamon filling over it, then top with remaining batter and filling. Swirl gently.

Bake for 40 to 45 minutes until golden brown and a toothpick comes out clean.

Let the bread cool for about 10 minutes before removing from pan.

For icing, whisk together powdered sugar, milk, and vanilla until smooth. Adjust thickness if necessary.

Drizzle icing over the cooled bread, slice, and serve warm.