Preheat oven to 325° F (160° C). Position rack in the center.
Line baking sheet with parchment paper. Set aside.
In a medium bowl, whisk together the dry ingredients (flour, baking powder and salt). Set aside.
In a large bowl, whisk the eggs together with both sugars, vanilla extract and limoncello (or lemon juice) until mixture thickens slightly and turns a lighter color.
Add the dry ingredients to the wet ingredients; stir with a wooden spoon until just incorporated.
Use a large spoon to scoop out the dough and place it on the parchment lined cookie sheet. Add another scoop adjacent to the batter on the cookie sheet and keep going until two logs are formed. Space the logs about 3 inches apart.With damp fingers, shape each section of dough into a log shape.
Bake for approximately 30 minutes or until light golden brown in color and firm. Allow to cool slightly for about 10 minutes.
Remove logs from baking sheet and transfer to cutting board.
Using a serrated knife, slice cookies at an angle about ½ - ¾ inch thick.
Place slices flat, back on the baking sheets, and return them to the oven for about another 10-15 minutes (the longer they stay in the oven, the crispier they get). Turn them over at the half way mark.
Place on a wire rack to cool completely before storing.
Notes
Lemon sugar can easily be made by pulsing the lemon zest from one lemon with ¾ cup of granulated sugar in a food processor.
Variations:
Orange Biscotti: Replace the lemon juice with 1 tablespoon of orange juice concentrate; replace the lemon zest with that of an orange.Spiced Biscotti: Eliminate the lemon zest. Add 1 teaspoon of Allspice. Chocolate Chip Biscotti Recipe: Replace the lemon zest with 1 tablespoon of orange zest. Add ½ cup of dark chocolate mini chocolate chips.Please keep in mind that the nutritional information provided below is just a rough estimate and variations can occur depending on the specific ingredients used.