01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 2-3 minutes.
03 - Beat in the egg, vanilla extract, and peppermint extract until fully incorporated and smooth.
04 - In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt until well blended.
05 - Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Avoid overmixing.
06 - Stir in the semi-sweet chocolate chips until evenly distributed throughout the dough.
07 - Drop rounded tablespoons of dough onto the prepared baking sheets, spacing about 2 inches apart to allow for spreading.
08 - Bake for 9-11 minutes, until the edges are set but the centers still look slightly soft.
09 - Let the cookies rest on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
10 - While cookies cool, beat the softened butter in a medium bowl until creamy and smooth, about 1-2 minutes.
11 - Gradually add the powdered sugar, mixing well after each addition. Add the milk, peppermint extract, and green food coloring. Beat until fluffy and adjust coloring as desired.
12 - Once the cookies are completely cool, spread or pipe the green frosting generously onto each cookie.