Pistachio Shortbread

Pulse pistachios in a food processor until finely ground but not a paste, about 30 seconds. Add flour, sugar, butter, salt, and vanilla; pulse until mixture is the consistency of cornmeal. With machine running, drizzle in 2 tablespoons ice water (a crumbly dough should form; do not overprocess). Transfer dough to a sheet of parchment paper and pat into a rectangle. Top with another sheet of parchment and roll out to a 12x8" rectangle. Transfer dough (in parchment paper) to a baking sheet and chill until firm, about 1 hour. Place racks in upper and lower thirds of oven; preheat to 350 °F. Remove top sheet of parchment paper from dough and discard. Cut dough lengthwise into 8 equal strips, then each strip crosswise into quarters, forming 32 rectangles. Place rectangles on 2 parchment-lined baking sheets, spacing 1" apart. Bake shortbread until golden brown, rotating sheets halfway through, 18-20 minutes. Transfer to wire racks; let cool. DO AHEAD: Shortbread can be made 5 days ahead. Store airtight at room temperature.