Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large mixing bowl, whisk together the flour, brown sugar, granulated sugar, baking powder, baking soda, salt, and ground cinnamon until well combined.
In a separate bowl, mix the melted butter, eggs, and vanilla extract until smooth.
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.
Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
While the bars are baking, prepare the glaze by whisking together the powdered sugar and milk in a small bowl until smooth.
Once the bars are done, remove them from the oven and let them cool in the pan for about 10 minutes. Drizzle the glaze over the warm bars.
Allow the bars to cool completely before cutting into squares and serving.