Line 2 baking sheets with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, brown sugar, and granulated sugar. Beat until well mixed, 1-2 minutes.
1 cup unsalted butter, 1 cup brown sugar, ½ cup granulated sugar
Beat in the eggs, vanilla extract, and peppermint extract until light and fluffy, about 2 minutes.
2 large eggs, 1 teaspoon pure vanilla extract, ½ teaspoon peppermint extract
In a separate medium bowl, whisk the flour, cocoa powder, baking soda, cornstarch, and salt together. Mix the dry ingredients into the wet just until combined. Do not overmix.
2½ cups all-purpose flour, ½ cup unsweetened cocoa powder, 1 teaspoon baking soda, 1 teaspoon cornstarch, 1 teaspoon kosher salt
Gently fold in all the chocolate chips and 1 cup of the chopped Andes mints.
1 cup semisweet chocolate chips, 1½ cups finely chopped Andes mints
Using a 2-tablespoon-sized cookie scoop, scoop cookies onto 1 of the prepared baking sheets. The cookies can be placed close to each other on the baking sheet in order to fit them all into the freezer in step 8. Use your hands to roll the cookies into smooth balls.
Press the remaining ½ cup of chopped Andes mints into the tops of each cookie.
Place the cookies in freezer for 10 minutes before baking. This helps prevent the cookies from spreading too much during baking. While the cookies chill, preheat oven to 350°F.
Take 6 cookies out of the freezer and place them on the second prepared baking sheet. The cookies should be at least 2 inches from each other on the baking sheet. Bake one tray of cookies at a time., leaving the other cookies in the freezer until you’re ready to bake them.
Bake each tray of cookies for 10-12 minutes, or until cookie edges seem set. The middles of cookies will still look slightly soft/undercooked and this is okay. They’ll finish baking on the hot tray when you remove them from the oven. Remove from oven and let cool completely on tray before transferring to a wire rack.