1 cup + 2½ Tbs unbleached all-purpose flour
¼ cup Dutch- processed cocoa
¾ cup unsalted butter, at room temperature
½ cup powdered sugar, sifted
1½ tsp pure vanilla extract
¼ tsp salt
¾ cup chopped chocolate chips
3 Tbs unsalted butter, cut into 6 pieces
¼ tsp pure peppermint extract
1. Sift flour and cocoa together into a medium bowl. With a hand mixer or a stand mixer fitted with the paddle attachment, cream butter and sugar on medium speed until light and fluffy, about 2 minutes. Add the vanilla and salt; continue beating until blended and smooth, about 1 minute more. Add the flour-cocoa mixture and mix on low speed until a soft dough forms, about 1 minute.
2. Chill the dough in the refrigerator until firm enough to roll into balls, 40 to 60 minutes (or longer).
3. Position a rack in the center of the oven and heat the oven to 350°F. Line two cookie sheets with parchment or nonstick baking liners.
4. Using your palms, roll heaping teaspoonfuls of the dough into 1-inch balls. Arrange them 2 inches apart on the lined sheets. With a lightly floured thumb or index finger-tip, press straight down into the middle of each ball almost to the cookie sheet to make a deep well. (Or use the end of a thick-handled wooden spoon.)
5. Bake one sheet at a time until the tops of the cookies look dry, 8 to 9 minutes. Gently redefine the indentations with the end of a wooden spoon. Let the cookies cool on the sheet for 5 minutes and then let them cool completely on racks.
6. For the filling: Put the chocolate and butter in a heatproof bowl. Microwave for 30 second bursts (stirring after each heating) until melted and smooth. Stir in mint extract. Let the filling cool, stirring occasionally, until slightly thickened and a bit warmer than room temperature, 30 to 40 minutes. Spoon the filling into a small pastry bag with a small plain tip. (Or use a small plastic bag and cut a tiny bit off a bottom corner of the bag.) Pipe the filling into the center of each cookie. Cool completely before serving or storing.