How to Make It
In a medium bowl, whisk the flour with the cocoa powder and baking soda.
In a large bowl, using a hand mixer, beat the softened butter with the granulated sugar and salt at medium speed until smooth, about 3 minutes. Reduce the speed to low and beat in the vanilla and egg white. Add the dry ingredients in 2 batches, beating until just combined.
Turn the dough out onto a work surface. Divide in half and form each piece into a disk. Sandwich 1 piece of dough between 2 sheets of parchment paper and roll out 1/8 inch thick. Transfer the dough and parchment paper to a baking sheet. Repeat with the remaining piece of dough and 2 more sheets of parchment paper; stack on the baking sheet. Freeze until firm, about 1 hour. Alternatively, refrigerate for 2 hours.
Preheat the oven to 350° and line a large rimmed baking sheet with parchment paper. Remove both sheets of parchment paper from 1 piece of dough and, using a 2-inch scalloped round cutter, stamp out cookies. Transfer to the prepared baking sheet, about 2 inches apart, and sprinkle generously with sanding sugar. Bake the cookies for 12 minutes, or until the edges are firm; rotate the baking sheet halfway through baking. Transfer to a wire rack and let cool completely. Repeat to bake the remaining piece of dough, rerolling any scraps.