1. Preheat oven 350F. Pulse flour, 2/3 cup sugar, espresso powder, baking powder, and salt in a food processor. Add butter and pulse until mixture is sandy. Transfer to a large bowl.
2. Whisk eggs, water, and vanilla in a small bowl; stir into flour mixture until a crumbly dough forms. Press together and knead to a moist, uniform dough. Knead in cashews.
3. Line a baking sheet with parchment paper. Divide dough in half and place on prepared baking sheet. Shape dough into two 11 by 2-inch logs, keeping 3 inches between logs. Brush each log with egg white and sprinkle 2 teaspoons sugar over top and sides of each. Bake for 30 minutes.
4. Cool biscotti for 5 minutes on baking sheet. Transfer logs to a cutting board and, cut 1/2-inch thick slices using a sharp serrated knife. Lay on baking sheet and bake, turning halfway through, for 25 minutes. Transfer to a wire rack to cool. Store in an airtight container for up to 2 weeks.
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