Sift together almond flour and powdered sugar and set aside.
Using a hand mixer, beat egg whites for 2 minutes until opaque and foamy (soft peaks). Slowly mix in granulated sugar a little at a time. Then beat on high for 2 to 5 minutes until stiff peaks form.
Add in peppermint extract and green food color gel. Beat for an additional 30 seconds.
Sift ⅓ of dry ingredients into egg whites, and, using a rubber spatula, gently fold in until almost incorporated. Repeat with another ⅓ of dry mix and then the remaining ⅓.
Once there is no remaining dry mix, continue to gently fold batter together until it becomes slightly runny, the texture of lava. Check consistency by trailing a ribbon of batter from the spatula over the surface of the rest of the batter. You should be able to complete a figure-8 if it is thinned enough.
Fit your pastry bag with a ½ inch round tip. Place bag inside a glass for stability, and fold top of the bag down over glass. This will help cleanly fill your pastry bag with batter.
Line 2 baking sheets with either parchment paper or silicone baking mats.
Pipe ¾ inch rounds with at least 1 inch of space between.
Firmly tap the baking sheet against the counter to bring air bubbles to the surface. Follow with a toothpick to pop additional subsurface bubbles and fill in holes.
Allow to dry at room temperature for 30 to 40 minutes until you can touch the top without batter adhering to your finger.
Preheat the oven to 315°F.
Bake both sheet pans for 6 minutes and then rotate the pans and bake an additional 6 minutes.
Allow the shells to cool on the baking sheet for 5 minutes before transferring to a cooling rack.
Peppermint Cocoa Buttercream
Whip softened butter until fluffy.
Add in powdered sugar and whip until incorporated. Then add in cocoa powder.
Thin with milk and whip until color lightens and mixture is airy.
Flavor with peppermint extract.
Fit a piping bag with a decorative tip or the same (clean) ½ inch round tip.
Final Construction
Turn half of the shells over so the feet face up. Pipe buttercream onto these shells.
Gently press the remaining shells feet side down onto the frosted shells to form little sandwiches.