1. Preheat the oven to 350°F (175°C) and grease a 9×13 inch baking pan.
2. In a large bowl, cream together the butter and granulated sugar until light and fluffy.
3. Add the mashed bananas, eggs, and vanilla extract, mixing until well combined.
4. In another bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon.
5. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
6. In a small bowl, mix together the brown sugar and cinnamon for the swirl.
7. Pour half of the batter into the prepared pan, sprinkle with half of the brown sugar mixture, then add the remaining batter on top.
8. Swirl the remaining brown sugar mixture into the top layer of batter with a knife.
9. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
10. For the glaze, whisk together the powdered sugar and milk until smooth and drizzle over the cooled cake.

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