In the top portion of a double-boiler, melt together the dark chocolate and butter. Remove from the heat and stir until smooth.
In a mixing bowl, whisk together the flour, cocoa, salt, baking powder, and baking soda. Set aside.
Whisk the eggs, brown sugar, granulated sugar, and vanilla into the melted chocolate mixture. In two batches, stir in the dry ingredients until just combined. Fold in the chocolate chips.
Wrap the dough in plastic and chill for at least 30 minutes.
When ready, pre-heat the oven to 350. Line a few baking sheets with parchment and set aside.
Roll the dough into 2-tablespoon balls (about 25 grams). Place them on the baking sheets, at least 1-inch apart. Bake for about 10 to 11 minutes, or until the edges are cooked and centers are slightly cracked. Cool on the baking sheet for 5 to 10 minutes before transferring to a cooling rack. Completely cool the cookies before filling with the peppermint buttercream (recipe to follow).
Sort the cookies into pairs and pipe the peppermint buttercream on the bottom halves. Gently sandwich the cookies together with the top halves.