Add the Thin Mint cookies to a food processor. Pulse until a fine crumb has happened. Sift the crumb with a fine mess strainer to remove any large pieces.
Line a baking sheet with parchment paper or silicon mat (aff. link).
Sift together the almond flour, powdered sugar, Thin Mint crumbs, and cocoa powder. If you can't find almond flour you can always make it with almonds and a food processor. Finely grind the almonds and sift through a fine mesh sifter. Toss aside any large pieces!
In a large mixing bowl, add in egg whites. Turn on high and beat until the egg whites form a soft peak meringue.
Add in the granulated sugar, salt, peppermint extract, and cream of tartar. Mix on high until a stiff peak meringue forms. I slowly but constantly add in the granulated sugar into the meringue instead of tossing it all in at once.
Very lightly and carefully fold in a third of the almond flour into the meringue with a spatula. Repeat with the rest of the almond flour mixture. All together you'll want to have about 59 turns of this mixture. If you don't mix enough the macaron shell will be extremely fluffy and cracked. If you over mix, the macaron shell will be extremely flat and might not form a foot.
Preheat oven to 300 degrees.
Fill a piping bag with the macaron shell filling. I just used a coupler on the end of my piping bag. Pipe macaron shells on the baking sheet.
If you want to add a little something to your macarons, sprinkle any extra cookie crumbs on top of the macaron shells. Remember to account for the 2 tablespoons for the frosting.
Let the macarons dry until they are no longer tacky to the touch. It will take 20 minutes to 1 hour.
Place in the preheated oven for about 10 minutes until they no longer stick to the parchment paper. The shells won't be shiny. They will be dull and may be a few dimples. It's from the crumbs.
Let the macaron shells cool.
In a large mixing bowl, add in the butter and mix until creamy.
Add in the powdered sugar and milk and mix until light and fluffy.
Add in the green gel paste color, peppermint extract and Thin Mint crumbs and mix until incorporated.
Place the frosting into a piping bag with a straight tip.
Match up the cooled shells that are like size. You won't have to worry about this if you use a macaron silicone mat with the template on them.
Pipe the frosting on top of the cooled shells. Be gentle to not crack the shells. Place on the top shell.