1. Preheat oven to 350ºF. Line a large baking sheet with parchment.
2. Combine flour, sugar, baking powder and salt in a large mixing bowl. Add 3 eggs and vanilla and beat with an electric mixer on low speed until just combined, about 30 seconds. Mix in cranberries and pistachios.
3. Turn dough out onto a lightly floured work surface and divide in half. Shape each half into a flat log about 14 inches long and 2 inches wide. (Dough will be sticky.) Place logs on prepared baking sheet several inches apart.
4. In a small bowl, beat remaining egg and brush over dough. Sprinkle with sanding sugar, if desired. Bake logs until they are firm to the touch, about 35 minutes. Place baking sheet on a wire rack and allow logs to cool completely.
5. Lower oven to 325ºF. Transfer logs to a cutting board and use a serrated knife to carefully cut each into 18 slices. Lay slices cut side down on baking sheet and return them to oven. Bake until cookies just begin to get crisp, about 8 minutes. Transfer cookies to wire racks and let cool completely.