In a small saucepan over low heat, melt chocolate & butter until smooth. The mixture will be THICK, but smooth. Set aside.
Place eggs, sugar & vanilla in the bowl of a stand mixer fitted with a paddle attachment & beat for 15 minutes or until pale & creamy.
Add flour, baking powder & chocolate mixture. Mix to combine. Scraping the bowl & paddle as necessary.
Scoop 2 tablespoonfuls for each cookie (I use a #50 scoop) onto a parchment lined baking sheet.
Bake at 350˚F for 10-12 minutes or until puffed & cracked. Allow to cool completely on trays.
While cookies cool, in a large bowl with a hand mixer, beat butter, powdered sugar & creme de menthe extract. You can add more mint flavoring until desired mint-iness, but don't add too much that it tastes like eating toothpaste. Add green food coloring & mix to desired color.
Put the inside of a coupler into a pastry bag & leave the band off. Fill with mint icing & swirl onto the underside of one brownie cookie. Top with a second brownie cookie, bottom side down on top of the icing. Twist & push with a little bit of pressure to make sure filling spreads to the edges without breaking the cookie. Sprinkle edges of icing with green sugar crystals. Repeat with remaining cookies.