01 - Preheat oven to 350°F. Line two large baking sheets with parchment paper.
02 - In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy, about 3 minutes.
03 - Beat in eggs one at a time, ensuring each is fully incorporated. Mix in vanilla extract and peppermint extract.
04 - In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until well blended.
05 - Gradually add the dry ingredients to the butter mixture on low speed, mixing just until combined. Fold in chocolate chips by hand.
06 - Drop rounded tablespoon portions of dough onto prepared baking sheets, spacing about 2 inches apart.
07 - Bake for 9-11 minutes until edges are set but centers appear slightly soft. Cool on baking sheets for 5 minutes.
08 - Transfer cookies to a wire rack and let cool completely before frosting.
09 - Beat the softened butter until creamy and smooth. Gradually add powdered sugar, mixing on low speed until incorporated.
10 - Add milk and peppermint extract. Increase speed to medium-high and beat until fluffy, 2-3 minutes. Add green food coloring and mix until uniform color achieved.
11 - Spread or pipe frosting onto completely cooled cookies. Let frosting set for 15 minutes before serving.