Preheat the oven to 325 (160 C) degrees. Line a large rimmed baking sheet with parchment paper.
Whisk the egg whites in mixing bowl of an electric mixer with ¾ cup sugar to form soft peaks. Set aside (or transfer to another bowl if you only have one mixing bowl).
Using the paddle attachment, beat the egg yolks with remaining ¾ cup of the sugar until pale and thickened, about 3 minutes. On low speed, mix in the melted butter, lemon extract and lemon zest.
Whisk the flour, baking powder and salt together in a medium bowl. Add half the flour mixture to mixing bowl and stir. Add the beaten egg whites, followed by the remaining flour and pine nuts. Stir on low speed until the dough is firm and slightly sticky.
Divide the dough in half and form each half into a log approximately 2 inches in diameter 12 inches long. Arrange the logs side by side on the baking sheet and bake 25 minutes, or until the logs are firm and form cracks on top.
Remove from the oven and cool the logs for about 10 minutes. Transfer to a cutting board with a wide spatula and cut into ½-inch wide diagonal slices with a sharp knife.
Arrange the slices cut-side up on the baking sheet and bake 6 minutes on each side or until light golden and crisp.
Biscotti keep well at room temperature for up to a week.
Freeze biscotti for 1 month. Defrost at room temperature before serving.