2 cups all-purpose flour
2/3 cup unsweetened cocoa powder
1/4 teaspoon fine salt
1 1/4 cups (2 1/2 sticks) unsalted butter, room temperature
1 1/2 cups granulated sugar
1 large egg
1 tablespoon pure vanilla extract
3/4 cup semisweet chocolate chips
1/2 teaspoon pure peppermint extract
Preheat oven to 350 degrees F. Set racks in upper and lower thirds. Line two baking sheets with parchment.
In a medium bowl, whisk together flour, cocoa powder, and salt. Set aside.
In a large bowl, beat 1 cup each butter and sugar on medium-high using an electric mixer until light and fluffy, about 3 minutes. Add egg and vanilla and continue beating to combine.
Gradually add flour mixture onto butter mixture and beat on low until combined.
Place 1/2 cup sugar in a small bowl. Roll dough into 1-inch balls, then roll in sugar to coat. Place balls, 1 inch apart, on prepared baking sheets. Bake for 5 minutes. Remove sheets from oven; with the small end of a melon baller or the bottom of a small round measuring spoon, make an indentation in center of each cookie.
Bake until cookies are just set but still look moist, about 4 minutes (do not overbake). Let cool on sheets on wire racks.
To make the filling, heat chocolate chips and 1/4 cup butter in the microwave for 10-second increments until melted; stirring ocassionally. Add in peppermint extract. Let cool until thick enough to pipe, about 5 minutes. Transfer mixture into a pastry bag and pipe out chocolate filling into cookie indentations. Alternatively, you can use a large zip-top bag and snip a 1/4-inch hole in one corner to fill the cookies.