In a medium bowl, whisk the flour, baking soda, cornstarch, and salt together. Set aside for now.
3 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon cornstarch, ½ teaspoon kosher salt
In a separate large bowl, using a hand mixer, beat the butter and sugar together until well combined and light in color, about 2 minutes.
1 cup unsalted butter, 1 cup granulated sugar
Add the eggs and vanilla into the butter mixture and beat until light and fluffy, about 1 minute.
2 large eggs, 2 teaspoons pure vanilla extract
Add the flour mixture into the butter mixture and beat on low just until combined. Do not overmix.
Divide the cookie dough into 5 separate bowls. This works out to be about 187 grams of cookie dough for each of the 5 bowls.
Sprinkle 1 tablespoon of the dry Jell-O powder into each bowl (one tablespoon of strawberry into one bowl of dough, one tablespoon of lime into a different bowl of dough, one tablespoon of orange into a different bowl of dough, etc.).
5 tablespoons dry Jell-O mix
Add food coloring into each bowl (red for strawberry, green for lime, orange for orange, yellow for lemon, and blue for berry blast).
Gel food coloring
Use a spatula to stir Jell-O powder and food coloring into cookie dough.
Line a baking sheet with parchment paper.
Using a 2-tablespoon sized cookie scoop, scoop balls of cookie dough onto the parchment lined baking sheet. The cookie dough balls will be about 30 grams each. You’ll end up with 6 cookies in each of the 5 colors. Use your hands to roll cookies into smooth balls.
Place the tray of unbaked cookies in the freezer for 15 minutes to help them firm up.
While the cookies chill, preheat oven to 350°F.
Line a second baking sheet with parchment paper, take some cookies out of the freezer, and place them on the baking sheet 3 inches apart. The remaining cookies should stay in the freezer until they are ready to be baked.
Place one tray of cookies into the preheated oven. Bake each tray of cookies for 12-13 minutes. The cookies will still look a bit soft in the middle, but they’ll continue to bake on the hot tray after you remove them from the oven.
Let cookies cool for 15 minutes on tray before removing to wire rack to cool completely.