In a large bowl, whisk together the flour, cocoa powder and salt. Set this aside.
In a separate large bowl, either using a hand mixer or a stand mixer with the paddle attachment, beat the butter on med-high speed until smooth and creamy, about a minute or two. Then beat in the sugar until fluffy and combined. Beat in the egg, vanilla and peppermint extract. Use a rubber spatula to scrape the sides of the bowl as needed.
Switch the mixer to low speed and add in the dry ingredients. Mix until just combined. Then flour your hands and a work surface lightly. Turn out the dough onto the floured surface and seperate the dough into two even amounts.
Roll the dough into two seperate logs, roughly 8 inches in length and about 2 1/2 in in diameter. Wrap each log tightly in plastic wrap and place them in the fridge for at least 4 hours and up to 5 days. You could also freeze the dough logs at this point, for up to 3 months. To thaw, place them in the fridge overnight, then continue on with the baking process from there.
When you're ready to bake, preheat the oven to 350F. Line a couple of large baking trays with either parchment paper or silicone baking mats.
Working one at a time, unwrap the dough log and slice into equally sized cookies. You should get about 11-12 per log.
Place the cookies on the prepared baking sheet, about 2 inches apart from each other. Bake for 11-13 minutes. Allow them to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
Once cooled, melt the 4 oz. semi-sweet chocolate according to its packages directions and drizzle over the top of each cookie. Allow the chocolate to set completely (about 30 or so minutes) before storing them.
Store in an airtight container for up to 1 week.
