Preheat the oven to 350°F. Spray an 8-inch or 9-inch square baking pan with cooking spray, making sure the corners are well coated.
In a medium bowl, whisk together 1 ½ cups all-purpose flour, baking powder, 1 teaspoon cinnamon, and salt until evenly combined.
In a large bowl, stir together the granulated sugar, vegetable oil, and egg until smooth and slightly glossy. Stir in the milk.
Add the dry ingredients to the wet mixture and stir just until no dry streaks remain. Do not overmix. Fold in the shredded zucchini until evenly distributed.
Spread the batter into the prepared pan in an even layer.
In a small bowl, stir together the brown sugar, remaining 1 tablespoon flour, remaining 1 teaspoon cinnamon, and melted butter until damp crumbs form.
Sprinkle the crumb topping evenly over the cake batter.
Bake for 30 to 35 minutes, or until the edges are set and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the cake cool for at least 20 minutes before slicing so the crumb sets and the topping firms slightly.
