Cream the soft butter, sugar, vanilla and salt together. Add in the cocoa powder and mix until the cocoa powder is integrated and the batter resembles a thick frosting.
Beat in the egg, scraping down the sides of the bowl.
Whisk the flour and cornstarch together and add to the bowl, mixing on low just until everything is well combined, but don't over-mix.
Divide the dough into 2 halves and flatten into disks. Roll out each disk between two large pieces of parchment paper. Chill the dough for 1 hour.
Set the oven to 350F.
Cut the dough into rounds with a 2" cookie cutter.
Place cookies on a baking sheet, an inch apart, and bake for 10 minutes. I like to bake one tray at a time for more even baking. Let the cookies cool on the pans.
Make the peppermint coating. I do this in 2 batches, one for the first half of cookies, and the second when needed for the rest. Chop half of the bar chocolate into very small, thin pieces. Place in a glass measuring cup or similar container along with half the oil. Microwave for 60 seconds, then stir until completely melted. You can put it back in the microwave for 15 second bursts if necessary to get all the chocolate melted. Stir in the peppermint extract.
Gently drop the cooled cookies, one at a time, into the coating. Drop face side down, then use a fork to flip the cookie to coat the bottom. Lift the cookie with your fork and let the excess chocolate drip off. Set the cookie down on parchment paper to harden. Repeat for the rest of the cookies, making a new batch of chocolate coating when needed.
The cookies will set up in 30 minutes to an hour. They can be kept at room temperature for a week, refrigerated, or frozen for longer storage.
Video
Notes
Be sure that your butter is truly at room temperature before you start. If your butter is cool the dough will not come together well.
Use the 'fluff and scoop' method to measure out your flour. That means fluff up the flour in the canister or bag to loosen it BEFORE you scoop or spoon out your measurement. Level the top of the measuring cup gently with the back of a knife to get the most accurate measurement.
*This recipe was first published in 2012, and has been completely re-tested and updated in 2024.