Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper and set it aside.

In a medium mixing bowl, add the softened butter and granulated sugar. Using a hand mixer or stand mixer on medium speed, cream them together for about 2 minutes, until the mixture looks light, smooth, and slightly fluffy. Do not overmix; the goal is softness, not excessive air.

Add the lemon zest directly into the butter and sugar mixture. Mix for about 20–30 seconds, just until the zest is evenly distributed and fragrant. This step helps release the lemon oils into the dough.

Add the egg, lemon juice, and vanilla extract. Mix on low speed until fully combined. Scrape down the sides of the bowl to ensure everything is evenly incorporated.

In a separate small bowl, whisk together the flour, baking powder, salt, and cornstarch if using. Slowly add the dry ingredients to the wet ingredients, mixing on low speed. Stop mixing as soon as the dough comes together. The dough should be soft and slightly thick, but not sticky.

Using a cookie scoop or tablespoon, portion the dough into 8 equal mounds. Place them on the prepared baking sheet, leaving space between each cookie. Gently press the tops very slightly to flatten just a bit; do not flatten completely.

Place the baking sheet in the preheated oven and bake for 9 to 11 minutes. The cookies should look set on top but remain very pale, with no golden browning on the edges. They may appear slightly underbaked in the center, which is correct.

Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes. During this time, they will continue to set while staying soft. After 5 minutes, transfer them carefully to a wire rack to cool completely.