01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy using an electric mixer.
03 - Beat in the eggs one at a time, then add vanilla and peppermint extracts, mixing well after each addition.
04 - In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt until evenly distributed.
05 - Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Avoid overmixing.
06 - Fold in the chocolate chips by hand until evenly distributed throughout the dough.
07 - Drop tablespoon-sized balls of dough onto the prepared baking sheets, spacing them about 2 inches apart to allow for spreading.
08 - Bake for 9 to 11 minutes until edges are set but centers remain slightly soft.
09 - Let cookies cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely before frosting.
10 - Beat the softened butter until creamy and smooth. Gradually add powdered sugar, mixing on low speed to incorporate.
11 - Add milk and peppermint extract. Beat on medium-high speed until smooth and fluffy. Add green food coloring a few drops at a time until desired shade is achieved.
12 - Once cookies are completely cool, spread or pipe frosting on top of each cookie. Allow frosting to set for about 15 minutes before serving.