Line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking soda, and cornstarch. Set aside for now.
3 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon cornstarch
In a separate large bowl, using a hand mixer, beat together the butter and sugar until mixed well and light in color, about 2 minutes.
1 cup salted butter, 1½ cups granulated sugar
Beat the eggs and vanilla into the butter mixture until light and fluffy, about 1 minute.
2 large eggs, 1 teaspoon pure vanilla extract
Add the flour mixture into the butter mixture and beat on low just until combined. Do not overmix.
Separate the cookie dough evenly into 2 separate bowls (about 530 grams per bowl).
In one bowl of cookie dough, add in the strawberry Jell-O powder and 1-2 drops of pink food coloring, if using. Fold by hand using a rubber spatula until the dough is evenly colored but not overmixed.
2 tablespoons strawberry Jell-O powder, Gel food coloring
In the other bowl of cookie dough, add in the lemon Jell-O powder, lemon zest, and 1-2 drops of yellow food coloring, if using. Fold by hand using a rubber spatula until the dough is evenly colored but not overmixed.
2 tablespoons lemon Jell-O powder, Gel food coloring, ½ tablespoon lemon zest
Using a 1-tablespoon-sized cookie scoop, scoop balls of cookie dough onto the prepared baking sheet. These cookie dough balls will be about 16-17 grams each, and you should end up with 32 of each color. Use your hands to roll these into round balls.
Take 1 ball of pink dough and roll it with 1 ball of yellow dough. Roll it together well so it turns into 1 round cookie dough ball (each cookie dough ball will be about 32-33 grams).
Place the tray of unbaked cookies in the freezer for 15 minutes to prevent spreading.
Meanwhile, preheat your oven to 350°F and line a second baking sheet with parchment paper.
After 15 minutes, take some cookies out of the freezer and place them on the second baking sheet. The cookies should be 3 inches apart from each other. Leave any remaining cookies in the freezer.
Place one tray of cookies into the oven to bake for 13-14 minutes. The cookies won’t look completely set yet, but they’ll continue to bake on the hot tray after you remove them from the oven.
Let the cookies cool for 15 minutes on the tray before removing them to a wire rack to cool completely. Repeat with the remaining trays of cookies until you have baked all of them.