1) Preheat the oven to 350°F. Lightly grease (or line with parchment) two baking sheets.
2) In a medium bowl, beat together the butter and sugars.
3) Add the egg, beating well and scraping the bottom and sides of the bowl.
4) Add the vanilla and peppermint, beating to combine.
5) Add the flour, cocoa, baking soda, and salt, beating to combine. Taste the dough; add more peppermint, if desired.
6) Stir in the chocolate chips.
7) Drop a few generous teaspoonfuls of cookie dough onto the prepared baking sheets, spacing them about 1 1/2" to 2" apart. Since these cookies are dark-colored, it's difficult to tell when they're done. We suggest you bake 2 or 3 cookies alone first, before baking a couple of baking sheets' worth. That way, you can make sure you bake them to just the right degree of doneness.
8) Bake the cookies for 12 to 13 minutes (if they're on parchment-lined baking sheets); bake a couple of minutes less if they're on dark sheets without parchment. Remove them from the oven, and transfer them to a rack to cool. If they're crunchy and you prefer soft, bake the rest of the cookies a shorter amount of time; if soft and you're looking for a crunchy cookie, you'll know you need to bake them longer.
9) Bake the remainder of the cookies, letting them cool right on the pans, or transferring to a rack if you need the pans.
10) Wrap airtight, and store at room temperature. These cookies are sensitive to humidity; if they're crunchy, they'll quickly soften during humid weather, if not wrapped securely.
Yield: about 5 dozen 2" to 2 1/2" cookies.