Preheat your oven to 350°F (175°C) and grease an 8- or 9-inch baking pan.

Finely shred one medium zucchini using a box grater or food processor and squeeze out excess liquid.

In a medium bowl, combine the flour, baking powder, cinnamon, and salt. Whisk thoroughly.

In a larger bowl, blend the granulated sugar, vegetable oil, and egg until smooth. Gradually stir in the milk.

Slowly add the dry ingredients into the wet mixture, stirring gently. Fold in the squeezed zucchini.

In a small bowl, combine brown sugar, flour, cinnamon, and melted butter to create the crumb topping.

Pour the cake batter into the prepared pan, sprinkle the crumb topping generously, and bake for 30-35 minutes.

Let the cake cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.