Preheat oven to 350˚F.
Line a 9x13 loaf pan with parchment paper; lightly spray with cooking spray and set aside.
Shred zucchini using the large holes on a cheese grater.
Transfer shredded zucchini to a kitchen towel; close the towel around the zucchini and squeeze out as much liquid as possible. Set aside.
Mash bananas in a large mixing bowl.
To the bananas, stir in melted butter, sugar and vanilla.
Add egg and stir until incorporated.
Add in the flour, baking soda, cinnamon and salt; gently stir until incorporated.
Fold in the shredded zucchini.
Pour batter into prepared loaf pan.
Bake for 45 to 55 minutes, or until a toothpick inserted in the center of the loaf comes out clean.
Remove from oven and set the loaf pan over a wire rack; cool for 1 hour.
Remove bread from the loaf pan and let stand on wire rack until completely cooled.
In the meantime, prepare the icing by combining the milk, vanilla and powdered sugar in a mixing bowl or a large glass measuring cup. Whisk the ingredients until completely blended. If you want a thicker icing, add more sugar. If you want a thinner consistency, add a little more milk.
Drizzle icing over cooled loaf and let stand until icing is set.
Cut, serve, and enjoy!