Preheat the oven to 325ᵒF.
Line a large baking sheet with parchment paper. Sift the flour, cornmeal, baking powder and salt in a large bowl. Using an electric mixer beat the eggs and sugar in another large bowl until pale yellow and fluffy, about 3 minutes. Add the orange and lemon zests to the dry ingredients. Add the flour mixture to the eggs and sugar 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 mounds spaced evenly apart.  Moisten your hands with water and shape the mounds into two 11 by 14-inch logs,  Bake until lightly brown, about 25 minutes.  Cool the logs for 5 minutes.  Using a serrated knife, cut the logs crosswise into 1/2-inch-thick diagonal slices.  Arrange the biscotti, cut side down, on the same baking sheet.  Bake until the biscotti are pale and golden, about 25 minutes.  Cool the biscotti on the baking sheet for 5 minutes.  Using a metal spatula, transfer the biscotti to a metal rack and cool completely.

Melt the chocolate chips in the microwave, stirring every 20 seconds, about 1-1/2 minutes total.  Dip one cut side of each biscotti into the melted chocolate.  Gently shake off the excess chocolate.  Place the biscotti, chocolate side up, on the baking sheet.  Refrigerate until the chocolate is firm, about 35 minutes.  Then lightly brush the cocoa over the chocolate on each biscotti.

The biscotti can be made ahead.  Store them in an air-tight container for up to 4 days, or wrap them in foil and freeze in resealable plastic bags up to 3 weeks.