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.
The recipe suggests letting the biscotti cool for about 10 minutes before slicing. You may find it better to slice it sooner because it may get crumbly the longer you wait. Photo by Sarah Hauser.