In a medium bowl, whisk flour, cocoa powder, baking soda, and salt.

In a separate bowl or the bowl of a stand mixer, whip butter and sugar on high speed until light and fluffy.

Mix in the egg, vanilla extract, and peppermint oil (if using).

Add the dry ingredients. Mix on low speed until just combined. (Mixture will seem dry.)

Add the whole milk, and mix on medium speed until the dough comes together.

With a wooden spoon, fold in the chopped mint chocolate, chocolate chips, and M&Ms. Reserve a few of each for decorating the tops of the cookies.

Cover bowl with clingfilm/plastic wrap and transfer to the refrigerator to chill for 1 hour.

While dough chills, preheat the oven to 375° F / 190° C. Line a rimmed half-sheet baking tray with parchment or a silicone mat.

With a 1 1/2-inch cookie scoop, divide the dough into equally-sized portions. (Not all of your cookies will fit on one tray; return remaining dough to the refrigerator until ready to use.)

Use your hands to roll each portion into a ball. Roll in green sugar and transfer to prepared tray, allowing a few inches in between for the cookies to spread as they bake.

With your fingertips, pat down the top of each ball to flatten slightly.

Press reserved chocolate and M&Ms into the tops of the cookies.

Bake in preheated oven for 10-11 minutes. Do not overbake.

If desired, reshape the cookies with a large round metal cutter.

Cool cookies on tray for 10 minutes, then transfer to a wire rack to cool completely.

Repeat with remaining dough.

Store leftover cookies in an airtight container at room temperature for up to five days. Chocolate mint cookies can also be layered between sheets of parchment and frozen in an airtight container until ready to use.