Preheat oven to 350°F. Line a baking sheet with parchment paper. Whisk flour and baking powder in a medium bowl. Using an electric mixer, beat sugar, butter, lemon zest, and salt in a large bowl. Beat in eggs, 1 at a time. Add flour mixture and beat just until blended. Stir in pistachios and cranberries. Form dough into a 13-inch long, 3-inch wide log on baking sheet. Bake until light golden, about 40 minutes. Cool for 30 minutes.
Place log on cutting board. Using a sharp serrated knife, cut log on a diagonal into 1/2 to 3/4-inch-thick slices. Arrange biscotti, cut side down, on baking sheet. Bake biscotti until they are pale golden, about 15 minutes. Transfer biscotti to a rack and cool completely. Stir chocolate in a bowl set over a saucepan of simmering water until chocolate melts. Dip half of the biscotti into melted chocolate. Shake off excess chocolate. Place biscotti on baking sheet for chocolate to set. Sprinkle with sugar crystals. Refrigerate until chocolate is firm, about 35 minutes. Store in an airtight container up to 4 days, or wrap them in foil and freeze in resealable plastic bags up to 3 weeks.
Tip: Itʻs really easy to make your own colored sugar crystals. Just put 1/2 cup granulated sugar in a resealable plastic bag. Add a few drops of food coloring (adjust food coloring according to desired color). Seal bag and knead gently until color is evenly distributed. Spread sugar in a thin layer on a baking sheet, breaking up any lumps. Dry thoroughly, about 15-20 minutes. Sift if needed.
Approximate Nutrient Analysis per serving (based on 20 servings):
250 calories, 13 g fat, 7 g saturated fat, 35 mg cholesterol, 100 mg sodium, 32 g carbohydrate, 1 g fiber, 21 g sugar, 4 g protein