Preheat oven to 180°C.
Beat the egg and egg white together.
Add the oil, butter, sugar and vanilla extract and mix well.
Add the nuts and mix well. Sift the dry ingredients together and add to the egg mixture.
Mix until a soft dough is formed.
Cover and refrigerate for at least 2 hours.
Divide the dough into 2 large, neat rolls and place on a large greased baking tray, leaving enough space between rolls for spreading.
Place baking tray in oven and reduce heat to 160°C.
Bake the rolls for 20 - 25 minutes or until quite firm and lightly browned.
Remove from oven and cut each roll diagonally into 15 mm wide slices.
Lay slices on their sides in baking tray and return to oven at 100°C.
Bake for another 20 - 30 minutes or until crisp and lightly browned.
Switch off the oven and allow biscotti to cool in the oven.
Store in an airtight container.