Preheat oven to 350 degrees, with racks in upper and lower thirds. In a medium bowl, whisk together flour, cocoa powder, and salt. In a large bowl, using an electric mixer, beat 1 cup each butter and sugar on medium-high until light and fluffy, 3 minutes. Add egg and vanilla; beat to combine. With mixer on low, gradually add flour mixture and beat to combine.
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 two parchment-lined baking sheets. Bake 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, 4 minutes (do not overbake). Let cool on sheets on wire racks.
In a microwave-safe bowl, microwave chocolate chips and 1/4 cup butter in 10-second increments until melted; stir in peppermint extract. Let cool until thick enough to pipe, 5 minutes. Transfer mixture to a large zip-top bag; snip 1/4-inch hole in one corner. Pipe chocolate into cookie indentations.
All That Glitters: For extra sparkle, roll the dough balls in sanding sugar instead of granulated before baking. Store cookies in airtight containers, up to 1 week.