Preheat the oven at 350 degrees Fahrenheit (180 degrees Celsius)
In a bowl, mix the olive oil with sugar until smooth. Add the eggs beaten and mix well.
In another bowl, mix the flours, the salt and the baking powder. Add the flour mixture to the olive oil mixture and mix well.
Add the sweet potato and the pistachios and blend.
Grease a large pan and shape the dough into logs. You can choose the size depending on how big you want your biscotti. The dough will be pretty soft and sticky. Put some oil on your fingers and form the logs as well as you can.
Bake in the oven for about 30-40 minutes, until firm and golden.
Remove from oven and place them on a cooling rack and let them cool for about 15 minutes.
Cut them in slices (about 1/2 inch) with a sharp knife. To prevent the dough from sticking on the knife and to make sharp cuts, dip your knife in hot water before each cut.
Put back in the oven and bake for another 12-15 minutes on each side, until they are crispy.
Let them cool completely (otherwise they will be soft the next day) and store them in airtight containers.
If they become soft the next day, you can crisp them in the oven for 5 minutes.