Pistachio Shortbread

In the bowl of a food processor, grind pistachios until fine but not powdery. Divide ground pistachios equally between sanding sugar and flour; set aside.

In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar on medium speed until fluffy, 3 to 4 minutes. Add vanilla and salt, and beat for 1 minute. Add yolks one at a time, and beat until evenly combined. Add flour and pistachio mixture, and mix until just combined. Remove dough, and form into a 4-by-6-inch rectangle; wrap in plastic, and chill for 1 hour.

Preheat oven to 350 degrees. Line two baking sheets with Silpats (French nonstick baking mats) or parchment paper, and set aside. On a lightly floured surface, roll out dough to a 1/8-inch thickness. Brush dough with the egg white to moisten. Cover evenly with pistachio-sanding-sugar mixture. Lightly press mixture into dough with the rolling pin. Using a 1 1/2-inch fluted square cookie cutter, cut cookies as close as possible to avoid waste. Place on prepared baking sheets.

Bake until edges are just lightly golden, about 18 minutes. Transfer cookies to a cooling rack. Store in an airtight container for up to 1 week.