Preheat your oven to 350°F (175°C). Grease and flour your loaf pan or line it with parchment paper for easy removal.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until well combined.

In a separate bowl, combine the milk, melted butter, and egg. Mix well until all ingredients are fully incorporated.

Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix, some lumps are okay.

In a small bowl, mix together the ground cinnamon and brown sugar to create a cinnamon filling.

Pour half of the bread batter into the prepared loaf pan, then sprinkle half of the cinnamon filling evenly over the top. Pour the remaining batter on top and finish with the rest of the cinnamon filling.

Using a knife or a toothpick, gently swirl the cinnamon filling into the batter to create a marbled effect.

Bake in the preheated oven for 30-35 minutes or until a toothpick inserted into the center comes out clean.

While the bread is baking, prepare the glaze. In a small bowl, mix the powdered sugar with 2-3 tablespoons of milk until smooth. Adjust the consistency by adding more milk if needed.

Once the bread is done baking, remove it from the oven and let it cool in the pan for about 10 minutes before transferring it to a wire rack.

Drizzle the glaze over the cooled bread and let it set for a few minutes before slicing.