Preheat oven to 350F. Line a 9x13 baking pan with foil and lightly grease. Set aside.
In a medium bowl, combine flour, baking soda, baking powder, salt, and cinnamon. Set aside.
In a separate large bowl, whisk mashed banana, eggs, oil, milk, and vanilla. Fold flour mixture and zucchini into liquid mixture until just combined.
Spread batter into prepared pan and smooth top. Bake cake at 350F 25-30 minutes or until a toothpick inserted in middle comes out clean. Cool cake completely on a wire cooling rack.
In a small bowl, whisk glaze ingredients until smooth. Spread over cooled cake and let stand 5 minutes for glaze to set. Slice cake into squares and serve. Enjoy!