1. Adjust oven racks to center and lower third of oven. Heat oven to 325 degree F. Grease a large cookie sheet. Combine flour, baking powder, ginger, and salt in a bowl.
2. Beat butter and sugar in a large mixing bowl on medium speed until light and fluffy. Beat in eggs, one at a time, scraping bowl with a rubber spatula after each addition. At low speed, beat in flour mixture. Stir in walnuts and crystallized ginger.
3. Divide dough into thirds. On a lightly floured surface, roll each piece into a 14x1-1/2-inch log. Arrange dough 2-1/2 inches apart on prepared sheet. Bake on the bottom oven rack until firm, about 30 minutes (the center of each log should spring back when gently pressed). Let cool 15 minutes.
4. Reduce oven to 275 degree F. Transfer biscotti to a cutting board and with a serrated knife, cut diagonally into 1/2-inch slices. Arrange slices, cut side down, on two ungreased cookie sheets. Bake 30 minutes, until golden, switching and rotating sheets after 15 minutes. Cool on wire racks. Makes about 54 biscotti.