In a stand mixer with a cookie paddle attached, cream the butter and sugar together on medium speed until the mixture is light and fluffy, about 2 minutes.
Add the cocoa powder, salt, and vanilla, and mix until incorporated. Add half of the flour and mix. Add the remaining flour, 1/4 cup at a time, until you get a pretty dry dough. You don't want the dough to be sticky at all, and it might even crumble just a little. Don't be worried about a dry dough. We want a cookie that's cripsy and doesn't spread when baked!
Divide the dough into 3 balls, and pat them together to form a disk. Cover the disks in plastic wrap and refrigerate for about 15 minutes. If it stays in the fridge longer, let it rest at room temperature for 15 minutes before rolling out. The butter makes the dough super stiff if it's too cold.
Preheat the oven to 350 degrees. Remove plastic from a disk and roll the dough out between 2 pieces of parchment paper or on a lightly floured surface. Roll the dough thin, 1/4-inch to 1/8-inch thick.
Use a 2-inch circle cutter to cut out the cookies. Place on a cookie sheet and bake for 8 to 9 minutes. It's super hard to tell when a near-black cookie is done. I baked mine for 10 minutes the first round, and they tasted burnt on the bottom, so you'll need to adjust as you go. Repeat with the rest of the dough. Place the cookies on a wire rack to cool.
After cookies have cooled completely, it's time to get dippin'! Chop up the chocolate. Put it in a microwave-safe bowl, and microwave it for 1 minute. Reduce the power to half, and continue to microwave in 30-second increments, stirring after each 30 seconds. Do this until the chocolate is smooth and melted. It's possible to scorch and ruin the chocolate in the microwave. If you are prone to this, use a double-boiler method instead. If the chocolate seems too thick, you can add a little shortening to thin it out. When the chocolate is melted, stir in the peppermint extract. I started with 1/2 teaspoon of peppermint extract and then added a few drops at a time until it was minty enough for me. You should be right around 1 teaspoon of extract.
Drop a cookie in the melted chocolate, and make sure all of it gets covered in the chocolate. Fish the cookie out with a fork. Tap the fork on the edge of your bowl to encourage extra chocolate to drip off. Place the chocolate-coated cookie on a piece of parchment or waxed paper until set.
Best if enjoyed after waiting 8 hours after dipping in chocolate.
Notes
If you can, make these a day in advance, and let them sit before digging in. They taste even better after the flavors have time to set in.
I hear they store well in the fridge and freezer like real Thin Mints, but mine didn't last that long.
This was a very fun recipe to do with the kids and made for a nice treat after the activity!
My favorite time of year for these cookies is summer. Freezing them ahead and biting into one straight from the freezer on a hot summer day is true luxury!