How to Make It
In a medium bowl, whisk together the flour, cocoa, baking powder, baking soda, and salt; set aside.
With an electric mixer, beat the butter and brown sugar on medium-high speed until pale and fluffy, 2 to 3 minutes. Add the egg yolks and vanilla and beat to combine. Reduce the mixer speed to low, gradually add the flour mixture, and mix just until combined (do not overmix). Gently fold in the pistachios.
Divide the dough in half; place each half on a sheet of parchment paper and form into a 2-inch-diameter log. Wrap the logs tightly and chill until firm, at least 2 hours and up to 3 days.
Heat oven to 350° F with the racks in the upper and lower thirds. Unwrap the dough and slice with a serrated knife into ⅜-inch-thick rounds. Place the rounds on the baking sheets, spacing them 1 inch apart, and sprinkle with the coarse sugar.
In 2 batches, bake the cookies, rotating the baking sheets halfway through, until the edges are set, 11 to 13 minutes. Cool slightly in the pan, then transfer to a wire rack to cool completely.
Chef's Notes
Storage suggestion: Keep the cookies at room temperature in an airtight container for up to 5 days.