Preheat oven to 350 degrees. Line a biscotti pan with parchment paper. Alternatively, line a cookie sheet with parchment paper.
In a bowl whisk together the flour, baking powder, salt, cardamom and lemon zest. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, place the butter and sugar. Cream together until light and fluffy. Scrape down the sides of the bowl. Add in the eggs one at a time, mixing in completely and scraping down the side of the bowl between each one. The batter may looked curdled at this point. Stir in the lemon extract.
Gently stir the flour mix into the butter mixture. Stir only until the flour is completely absorbed. The batter will be thick.
Spread the batter into a biscotti pan, or shape into a 12x5 log on the cookie sheet. Bake for 35-45 minutes. The cookie sheet loaf will likely be done closer to the beginning of the time frame, while the biscotti pan should take the entire time. The dough will be light brown and firm to the touch. Remove the biscotti loaf from the pan and allow to cool for 20 minutes.
Lower oven temperature to 300 degrees.
When the dough is cool enough to handle but not completely cool it is easier to slice. Slice the biscotti loaf into approximately 1/2 inch biscotti. It should yield approximately 17-20 biscotti.
Place the biscotti onto a cookie sheet, evenly spaced, laying on their sides. Bake for 25 -30 minutes, until lightly browned and firm to the touch. For more even browning, you can flip the biscotti halfway through the cooking time.
Allow biscotti to cool completely before glazing.
For the Glaze
In a small bowl, stir the lemon juice into the powdered sugar. It may take a minute for the powdered sugar to absorb all the lemon juice. Stir until all the lumps are out and the glaze is smooth. Transfer the glaze to a ziploc bag, and seal the bag. Cut a tiny triangle off one bottom corner of the bag. Glaze the biscotti in a zig-zag pattern. Do not put the glaze on too thick or it will not harden properly. Sprinkle additional zest on if desired while glaze is still wet.
Allow the glaze to completely harden, this will take several hours. After the glaze has hardened, transfer the biscotti to a covered container. These will keep for several days.
This can also be made with a hand mixer, it will take longer to cream the butter and sugar. Also the batter is so thick, it may be easier to stir the flour in with a spoon.
*The lemon glaze is tart, you can substitute up to 1 tablespoon of milk for the lemon juice if desired.
*I often cook my biscotti standing up, so they brown evenly without flipping.
*The cardamom is optional.