Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
In the bowl of your stand mixer, cream together butter and both sugars for 2–3 minutes until light and fluffy.
Add eggs, vanilla, and peppermint extract (or emulsion) and mix until smooth.
In a separate bowl, whisk together flour, cocoa powder, cornstarch, baking soda, baking powder, and salt.
Gradually add the dry ingredients to your mixer, scraping down the sides as needed, until fully combined.
Gently fold in chocolate chips and crushed peppermint.
Scoop dough (about 2 tablespoons per cookie) onto prepared baking sheets, leaving space for spreading.
Bake for 9–11 minutes, or until the edges are set but the centers are still slightly soft.
Let cookies cool on the baking sheet for a few minutes before transferring to a wire rack.
(Optional: Sprinkle extra crushed peppermint on top while warm for that bakery-style finish!)