Preheat the oven to 325°F. Line a large baking sheet with parchment paper. In a large bowl, whisk together the flour, cornmeal, baking powder, and salt. In another large bowl, beat the sugar and eggs with an electric mixer until pale yellow and fluffy, about 3 minutes. Mix in the orange and lemon zests, then the flour mixture, and beat just until blended; the dough will be soft and sticky. Let stand for 5 minutes.

Using a rubber spatula, transfer the dough to the prepared baking sheet, forming two equal mounds spaced evenly apart. Moisten your hands with water and shape the dough into two 11x4-inch logs. Bake until the logs are lightly browned, about 35 minutes. Cool for 5 minutes. Using a serrated knife, cut the logs crosswise into half-inch-thick diagonal slices. Arrange the biscotti cut side down on the same baking sheet, and bake until the cookies are pale golden, about 25 minutes. Let cool before serving.

Makes 3 dozen cookies.