Wash and trim the zucchini, grate on the large holes, then place the grated zucchini in a clean kitchen towel or fine-mesh sieve and press or squeeze to remove excess liquid until it measures about 1/3–1/2 cup.

Preheat oven to 350°F and line baking sheets with parchment paper. In a bowl, cream the softened butter with the brown sugar and granulated sugar until light, about 2–3 minutes. Beat in the egg and vanilla until combined.

Whisk together the flour, cornstarch, baking powder, baking soda, and salt in a separate bowl. Gradually add the dry ingredients to the butter mixture and mix until just combined. Fold in the drained zucchini and chocolate chips; do not overmix.

Cover the bowl and chill the dough for 20–30 minutes to firm up the butter. Portion dough with a 1.5-tablespoon scoop or rounded tablespoon onto prepared sheets, spacing 2 inches apart, and press a few extra chocolate chips on top if desired.

Bake at 350°F for 10–12 minutes, rotating pans halfway through; edges should be set and lightly golden while centers remain slightly soft. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.