Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, whisk together the flour, brown sugar, baking powder, baking soda, salt, and ground cinnamon.
In another bowl, combine the melted butter, milk, egg, and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Spread the batter evenly in the prepared baking dish.
In a small bowl, mix the powdered sugar with 1 tablespoon of milk until smooth. Drizzle this icing over the top of the batter.
Use a knife to swirl the icing into the batter gently, creating a marbled effect.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for about 10 minutes before cutting into squares.