In a large mixing bowl beat the granulated sugar and butter until fluffy. Beat in the egg and vanilla until combined. In a separate small bowl whisk together the flour, salt, cocoa and baking powder. Add the flour mixture to the butter and beat until combined. Transfer the dough to a bag or cover with plastic wrap. Refrigerate for at least an hour.
Preheat oven to 350 degrees. Roll dough out to 1/8 inch thick on a large piece of parchment or wax paper. Using a cookie or biscuit cutter, cut dough into 2 inch rounds. Bake on a parchment or Silpat lined baking sheet for 8-10 minutes. Since the cookies are a very dark chocolate it may be hard to tell if they are done. If you start to smell chocolate, the cookies are done. Remove to a wire rack to cool.
Melt 1 cup of chocolate chips in a deep 2-cup measuring cup. I always use the microwave melting instructions on the individual bag or label. Melt the chips at 50% power for 1 minute, then stir. Repeat using 50% power at 30 second increments until almost smooth. Stir until all chips are melted and add 1/4 teaspoon of peppermint extract. Taste the chocolate and increase or decrease the extract depending on your preference.
Dip a completely cooled cookie in the chocolate coating both sides. Using a large serving or meat fork, hold the chocolate covered cookie over the melted chocolate tapping gently to remove the excess. Slide the Thin Mint onto wax paper. Cool until the chocolate is set and refrigerate or freeze until ready to serve!