Prep Your Oven & Pans: Begin by preheating your oven to 400°F (200°C). Line two baking sheets with parchment paper; this helps prevent sticking and promotes even baking for perfect cookies.
Cream the Cold Butter & Sugars: In a large mixer, combine the cold, cubed butter with both the packed brown sugar and white granulated sugar. Beat them together on medium-high speed for 3-4 minutes until the mixture is light, fluffy, and creamy. Using cold butter and beating it well helps create thick, chewy cookies.
Mix in Wet Ingredients: Add the room-temperature eggs and vanilla extract to the creamed butter and sugar. Continue to beat until everything is well combined, scraping down the bottom and sides of the bowl to ensure all ingredients are incorporated evenly.
Gently Fold in Dry Ingredients: Gradually add the all-purpose flour, cornstarch, baking soda, and salt to the wet mixture. Mix on low speed just until the dry ingredients are fully combined and a soft dough forms. Be careful not to overmix, as this can make your cookies tough.
Stir in Festive Add-ins: By hand, gently fold in the Andes peppermint crunch pieces, the chunky crushed Oreos, and the semisweet chocolate chips. Distribute these delicious additions evenly throughout your cookie dough for bursts of flavor in every bite.
Chill the Dough (Key Step!): For the best texture and to prevent your cookies from spreading too much, cover the dough and chill it in the refrigerator for at least 2 hours. Chilling overnight is even better for richer flavor and a super chewy result!
Portion & Bake: Once chilled, use a large 1/4 cup cookie scoop to place dough balls onto your prepared baking sheets, leaving a few inches between each for spreading. Bake in the preheated oven for 10-12 minutes. Look for edges that are just set and slightly golden, while the centers still look a bit soft – they'll continue cooking as they cool. Avoid putting dough on warm baking sheets.
Cool & Enjoy: Remove the cookies from the oven and let them cool on the baking sheets for at least 5 minutes to set. Then, transfer them to a wire rack to cool completely. Get ready to enjoy these delightfully chewy, minty-chocolate Christmas treats!