Preheat oven to 375 degrees. Line a baking sheet with parchment paper or a silicone mat. If you do not have either of those, simply spray the baking sheet lightly with non-stick spray.
In a large bowl, beat butter with an electric mixer (or use a stand mixer) on medium to high speed for 30 seconds. Add the brown sugar and combine well.
Add the baking powder and salt. Beat until combined, scraping side of bowl as needed.
Add in the eggs and beat until combined.
Add in the cocoa powder and blend well, scraping sides of bowl as needed.
Add in the flour using the mixer, a little at a time.
Using a wooden spoon, fold in the hazelnuts and 4 ounces of bittersweet chocolate with a wooden spoon.
Divide dough in half and shape each portion into a 9-inch-long roll. Place rolls 4 inches apart on prepared cookie sheet; slightly flatten each roll to 2 1/2 inches wide.
Bake in preheated oven for 20 to 25 minutes or until a toothpick inserted near centers comes out clean. Cool on cookie sheet for 45 minutes (yes this long so that that chocolate has a chance to harden slightly. It will make it easier to cut the rolls into slices.)
Reduce oven temperature to 325 degrees.
Cut each roll diagonally into 1/2-inch-thick slices, slicing carefully using a serrated knife. Place slices, cut sides down, on cookie sheet.
Bake in 325 degree oven for 14 – 18 minutes, turning the slices over halfway between cooking time. You want the biscotti to be dry and crisp. I like to take mine out of the oven after 16 minutes. Transfer the biscotti to wire racks and let cool.