Preheat oven to 350ºF. If you have 2 ovens, use both. If you have one oven you will need to switch the trays halfway through the cooking time.
In a medium size bowl, mix together (I use a whisk), the flour, cocoa, baking powder, and salt, set aside.
With the whisk attachment of your electric mixer, cream the butter, add oil, and add sugars. Make sure you scrape down the sides so that everything mixes together.
Add eggs one at a time, add vanilla.
Switch to the paddle on your electric mixer. Add in 2 or 3 stages the flour mixture to the butter mixture, slowly. Do not overmix!
Add nuts and chocolate, Do not overmix!
Split the dough in half. Form 2 logs across the length of 2 baking sheets (1 log on each sheet), you can do this on the diagonal also.
Bake for 20 minutes. Remove from oven (leave the oven on). Let cool on the baking sheets for about 15-20 minutes. Transfer the logs carefully to a cutting board. Slice, on a diagonal, ½ -¾ inch slices. Place each piece back on the baking sheet lying on their sides. Dry the biscotti out in the oven for 15 minutes (less time if you like then chewier).
