Preheat oven to 325F
Cream the butter, sugar, salt until light and fluffy and very pale yellow
Add the vanilla extract, lemon zest, juice, and eggs, one at a time, mix until incorporated
Add both flours and baking powder, and nuts, mix until incorporated
Move dough onto a floured surface, and divide into 3 equal parts, the dough is sticky so use plenty of flour
Form into 3-inch-wide strips, ½ inch tall and tapered on the sides, 'biscotti shaped'
Refrigerate dough for 30 minutes before baking
Bake for approximately 15 minutes, until just lightly browned
Let cool, then slice diagonally into 1/2-inch (1-cm) wide biscotti using a serrated knife
Lay biscotti on a cooking sheet and bake at 325 for 15 minutes, turn the biscotti over and bake for another 10-15 minutes until golden brown and crunchy