Grate the zucchini and squeeze out excess moisture using a clean kitchen towel or cheesecloth.

Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or a silicone baking mat.

In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.

In a large bowl, beat together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

Stir in the shredded zucchini until evenly distributed in the mixture.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in any optional mix-ins like chocolate chips or nuts.

Use a cookie scoop or tablespoon to drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are lightly golden and the centers are set.

Allow the cookies to cool on the baking sheet for 3-5 minutes before transferring them to a wire rack to cool completely.

In a medium bowl, beat together the softened cream cheese and butter until smooth. Gradually add the powdered sugar, beating until well combined. Stir in the vanilla extract. Add milk, one tablespoon at a time, until the icing reaches a spreadable consistency.

Once the cookies are completely cooled, spread a generous layer of cream cheese icing on top of each cookie. Garnish with additional toppings if desired, such as sprinkles or a sprinkle of cinnamon.