Preheat and Prepare: Preheat your oven to 350°F (163°C). Line baking sheets with parchment paper.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking soda, salt, and cornstarch. Set aside.
Cream Butter and Sugars: In a large mixing bowl, use a hand mixer or stand mixer to beat the softened butter, brown sugar, and granulated sugar until smooth and fluffy (about 2 minutes).
Add Eggs and Vanilla: Mix in the vanilla extract, then beat in the eggs one at a time, ensuring each is well incorporated.
Color the Dough: Add the blue and violet gel food coloring, mixing until the color is evenly distributed (see notes for coloring tips).
Incorporate Dry Ingredients: Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined.
Fold in Chocolate Chips: Stir in the semi-sweet and white chocolate chips, reserving about ⅓ cup of each for topping.
Portion the Dough: Use a 2 Tbsp cookie scoop to portion the dough onto the prepared baking sheets, spacing cookies about 2 inches apart.
Decorate Before Baking: Press a few of the reserved chocolate chips, Oreos, and Chips Ahoy pieces onto the tops of each cookie. Do not add the candy eyes yet.
Bake: Bake for 9 minutes, or until the cookies are set in the center but still slightly soft.
Final Decorations: Immediately after removing from the oven, press additional chocolate chips and candy eyes onto the cookies if desired.
Cool: Let the cookies rest on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.