How to Make It
In a large bowl, whisk the flour with the cocoa powder, baking soda and salt. In a stand mixer fitted with the paddle, beat 2 sticks of the butter with the brown sugar at medium-high speed until fluffy, about 3 minutes. Beat in the dry ingredients at low speed until just combined. Stir in the chocolate and vanilla. Divide the dough in half and press into disks. Wrap in plastic and refrigerate for 1 hour.
Preheat the oven to 375°. Line 3 baking sheets with parchment paper. Roll out the dough into a 1/4-inch-thick round. Using a 2-inch round cookie cutter, cut out cookies. Transfer to the prepared baking sheets, spacing them 1 inch apart. Bake for 6 minutes. Transfer the baking sheets to wire racks and let cool completely.
Meanwhile, in a stand mixer fitted with the paddle, beat the remaining 1 stick of butter with the confectioners’ sugar and peppermint at medium-high speed until white and fluffy, about 3 minutes. Spoon a heaping teaspoon of the filling onto the flat sides of half of the cookies. Sandwich with the remaining cookies and serve.