In a large bowl beat together butter and sugar.  Add eggs, lemon peel and vanilla.

In a separate bowl stir together flour, salt and baking powder.  Gradually add flour mixture to egg mixture.  Stir in pistachio nuts.  Turn dough onto lightly floured surface and divide dough into two equal size rectangular pieces.  Place dough onto lightly buttered cookie sheet.  Flatten tops slightly so they are about 2-1/2" wide.

Bake @350 Deg. F. for 30 minutes (approx.) or until lightly browned and firm to the touch. When biscotti are cool cut into 1/2" thickness.
 
Double baked biscotti: Reduce oven temperature to 325 Deg. F. and lay biscotti cut side down on cookie sheet for an additional 5 minutes.  Remove sheet from oven and turn biscotti over and bake for an additional 5 minutes until biscotti are firm and crisp. 

Frosting:
Beat together powdered sugar and lemon juice until smooth.  Drizzle over tops of  cooled biscotti.