Preheat oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
In a separate bowl, combine milk, oil, and egg. Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Pour half the batter into the prepared loaf pan. Sprinkle half the brown sugar and cinnamon over the batter.
Add the remaining batter and top with the remaining brown sugar and cinnamon. Swirl lightly with a knife to create a marbled effect.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
For the glaze, mix powdered sugar and milk until smooth. Drizzle over the cooled bread before serving.