Line ex-large cookie sheet with parchment paper or lightly grease, set aside. In a large mixing bowl beat butter with electric mixer on medium to high speed for 30 seconds, add sugar, baking powder & salt, beat until combined, scraping sides of bowl occasionally, beat in eggs & vanilla or lemon extract if you like it a little more lemony. Then beat in lemon peels & as much of the flour with the mixer, then using a wooden spoon stir in remaining flour and nuts. On a lightly floured surface divide dough into 3 equal portions , shaping into a 8" long loaf. Flatten loaf until 2 1/2" wide. Place at least 3" apart on prepared cookie sheet. Baking in oven at 375F for 20 - 25 minutes or until golden brown and tops are cracked.( loafs will slightly spread ). Cool on cookie sheet for 30 minutes. Transfer loafs to cutting board cut each loaf diagonally into 1/2" slices. Place slices cut side down on the same cookie sheet, bake in oven at 325F for 8 minutes more, then turn slices over and bake 8 - 10 minutes more or until dry and crisp. Transfer to a cooling rack until cool. In a small mixing bowl stir together powder sugar, lemon peels and adding just enough lemon juice 1-2 tsp to make icing a drizzle consistency. Dip each end of the biscotti into the icing or drizzle with icing. Makes 36 biscotti.