Line a baking sheet with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar, and granulated sugar together until well mixed, about 2 minutes.
¾ cup unsalted butter, 1½ cups brown sugar, ¼ cup granulated sugar
Add eggs and vanilla into the butter mixture and beat on high until fully incorporated, about 1 minute.
2 large eggs, 1 tablespoon pure vanilla extract
In a separate bowl, stir the flour, baking soda, cornstarch, and salt together.
3 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon cornstarch, ½ teaspoon kosher salt
Add the flour mixture into the wet mixture and beat on low just until combined. The dough will be very thick, but it shouldn’t be crumbly.
Gently fold in 1 cup of the chocolate chips by hand. Put the remaining 3 tablespoons of chocolate chips aside for now.
19 tablespoons semisweet chocolate chips
Separate cookie dough equally into 6 bowls. This works out to be around 200 grams of dough per bowl.
Color one bowl red, one orange, one yellow, one green, one blue, and one purple. Stir the colors into the dough well so each color is fully combined into the dough.
Gel food coloring
Take about a teaspoon of each color of cookie dough, and roll them together in your hands to create one cookie dough ball. Each cookie dough ball will be around 38 grams. The cookie dough ball will look like a tie-dye style of swirled colors.
Using the remaining 3 tablespoons of chocolate chips, press a few into the top of each unbaked cookie.
Place the tray of cookies into the freezer for 30 minutes before baking. This is an important step to help prevent the cookies from spreading too much during baking.
While the cookies chill, preheat oven to 350°F.
On a second baking sheet lined with parchment paper, place a few chilled cookies about 2 inches apart. Bake only 1 tray of cookies at a time.
Bake chilled cookies for 14 minutes, then remove from the oven and let cool completely on the baking sheet.