3 large eggs, separated
Preheat the oven to 325 degrees F. Butter or line a baking sheet with parchment. Using the paddle attachment on your mixer, or by hand with a wooden spoon, in a large bowl beat the egg yolks with 3/4 cup of the sugar until light and the sugar is dissolved. Stir in the melted butter, the almonds, Grand Marnier, and orange zest. In another bowl, using the whisk attachment, beat the egg whites until they just begin to form peaks and gradually beat in the remaining 3/4 cup sugar until the whites form stiff peaks.
In a medium bowl, sift together the flour and baking powder. Fold one-third of the flour into the egg yolks, then fold one-third of the egg whites in. Repeat, alternating, until well combined. The dough will be form and slightly sticky. If the dough is too soft, add more flour. With floured hands, divide the dough into two logs about 2 inches in diameter. Arrange the logs on the prepared baking sheet and bake for 20 to 25 minutes or until they are lightly brown and firm to the touch. Remove from the oven and set the baking sheet on a rack for 10 minutes. On a cutting board, cut the logs diagonally into slices 1/2 inch wide. Put the slices on the baking sheet and bake for 5 to 7 minute on each side until the biscotti are very lightly browned and crisp. Cook on racks and store in airtight container.