Adapted from POPSUGAR
Preheat oven to 350ºF. Line two baking sheets with parchment paper or a Silpat.
Add butter to a large mixing bowl and use a hand or stand mixer to cream on low. Gradually incorporate powdered sugar, mixing on low until combined with butter. Add egg whites in several increments, continuing to mix on low. Add vanilla, followed by flour in several increments (until just combined; don’t over mix the flour!).
Use a piping bag (or a large freezer bag–that’s what I did) with a small icing tip. I purchased this set and used one of the smaller plain ones. Make sure you only cut a tiny hole in the corner of the plastic; it will expand. Spoon large amounts of batter into the bag, and pipe batter into thin ovals (the cookie shape), about 2″ apart. The POPSUGAR video is helpful if you’ve never piped before!
Bake cookies for about 10 minutes until golden around the edges. Cool on a cookie rack (the cookies should be a little bendy).
In a small saucepan over medium heat, melt butter and add the cream. Pour chocolate chips and powdered sugar into a separate heatproof bowl.
As soon as cream mixture begins to bubble, remove from heat and pour into bowl of chocolate chips. Let it sit for 20 seconds, and then stir until chocolate chips melt completely and mixture begins to thicken. Stir in the crushed peppermint and let the ganache cool completely.
Spoon a large dollop–about a tablespoon–of ganache on one cookie and gently press a second cookie on top so the icing spreads evenly up to the edges. Repeat until all cookies are assembled.
Store in an airtight container for 3-5 days.