Beat the butter, confectioners’ sugar, and 1 teaspoon peppermint extract in a stand mixer fitted with a paddle attachment until smooth.
Add the flour, cocoa powder, and salt. Beat to combine.
Transfer the dough to a sheet of parchment paper.
Cover the dough with an additional sheet of parchment paper, then roll it out to ¼-inch thickness.
Transfer the parchment-covered dough to a baking sheet and chill at least 15 minutes, or until firm.
Preheat the oven to 375 F.
Cut out cookies using a 1 ¼- or 1 ½-inch round cutter and transfer to a lined baking sheet.
Roll and repeat the process with the dough scraps, until you have roughly 56 cookies.
Bake the cookies for 10-12 minutes, or until fragrant and firm. Optional: Before baking, poke holes on top of the cookies using a lollipop stick or skewer for a similar look to official Thin Mints.
In a microwave-safe bowl, heat the chocolate chips and oil until mostly melted.
Add the remaining ½ teaspoon peppermint extract to the chocolate and stir until fully melted and smooth.
Use a fork or dipping tool to dip a baked cookie into the chocolate coating, tapping and wiping off the excess on the bottom.
Repeat with remaining cookies, placing them back onto the lined baking sheet after dipping.