In a large bowl, whisk together the lemon zest, 1 cup of the granulated sugar, and light brown sugar until well-combined. Cream in the butter until fluffy. Whip in the vanilla extract, egg, lemon juice, and yellow food coloring, as desired. (I typically use about 10 to 12 drops.)
In a separate bowl, whisk together the flour, salt, baking soda, baking powder, and cornstarch. Add the dry mix to the wet ingredients and stir until fully combined, being careful not to over-mix. Cover and refrigerate for 1 hour.
Preheat oven to 350°F. Line two baking sheets with parchment paper. Pour 1/2 cup granulated sugar on a small plate.
Roll 2 tablespoons of dough into a ball and roll in granulated sugar. Place on baking sheet and repeat with remaining dough. Be sure to leave space surrounding the cookies for them to spread!
Bake for about 10-14 minutes, rotating half-way through, or until the cookies have spread out and the edges are golden, but the centers of the cookies still look soft, puffy, and undercooked. Every oven is different, so I recommend starting with just one or two cookies on the tray to see what baking time works best for you!
Let the cookies cool on the baking sheets until the cookies are firm enough to remove, about 15 minutes. Repeat with remaining batches, until all cookies are baked. Be sure not to let the trays cool completely before putting any more batches of cookie dough on them, as warm trays will cause the cookies to spread out too much.