Preheat oven to 325 degrees. Butter 2 baking sheets. Beat sugar and butter in a large bowl until blended. Add eggs, one at a time, beating well after each. Beat in orange peel and vanilla. Stir flour, baking powder, cinnamon and salt into a medium bowl. Add dry ingreadients to butter mixture; mix just until incorporated.
Divide dough in half. Place each piece on prepared sheet. With lightly floured hands, form each half into 3 inch wide by 3/4 inch wide hight log. Bake until dough logs are firm to the touch, about 35 minutes.
Transfer logs to work surface. Using serrated knife, cut on a diagonal 1/2 inch thick slices. Arrange cut sides down on baking sheets. Bake about 12 minutes. Turn biscotti over; bake about 12 minutes longer. Transfer to rack and cool.
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