01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat softened butter with granulated sugar and brown sugar until light and fluffy.
03 - Add eggs one at a time, beating well after each addition. Mix in vanilla and peppermint extracts.
04 - In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.
05 - Gradually add dry ingredients to wet mixture, mixing until just combined. Fold in chocolate chips.
06 - Drop tablespoon-sized dough balls onto prepared baking sheets, spacing 2 inches apart.
07 - Bake 10 to 12 minutes until edges are set and centers remain soft. Cool on sheets 5 minutes, then transfer to wire racks to cool completely.
08 - Beat softened butter until creamy and smooth.
09 - Gradually incorporate powdered sugar, milk, peppermint extract, green food coloring, and salt. Beat until fluffy, adding more milk if needed for spreadable consistency.
10 - Spread generous layer of green mint frosting onto completely cooled cookies.