Preheat oven to 275 degrees. In the bowl of an electric mixer, beat together the butter and sugar on medium-high speed until light and fluffy, about 2-3 minutes. Add the Nutella and beat until well blended. Reduce the mixer speed to medium-low and add the eggs one at a time, scraping down the sides of the bowl after each addition as needed. Add the vanilla and beat well.
In a medium bowl, whisk together the flour, baking powder, and salt. With the mixer on low, add the dry ingredients slowly until just incorporated. Add the chopped hazelnuts and chocolate and mix with a rubber spatula.
Divide the dough into two and turn onto a lightly floured surface. Squeeze the roll into a roll about 13 inches long and transfer to a parchment paper-lined cookie sheet. Gently flatten the rolls until they are about 2″ wide. Make sure to leave 4″ of space between the two rolls.
Bake for 30-35 minutes so they are firm and lightly golden brown. Transfer logs to a cooling rack and allow to cool to room temperature. Once they’re cooled, use a sharp knife to slice 1/2″ pieces on a slight diagonal. Place the sliced biscotti cut side down back onto the cookie sheets and bake them for another 30-40 minutes. Transfer to a cooling rack to cool completely.
*To make skinning the hazelnuts easier, boil them for a few minutes, then use a towel to rub the skins off. Then dry them with paper towels and toast them in a dry skillet until fragrant and golden brown.
Melt the chocolate in the microwave in 30 second intervals, mixing vigorously between each interval. Once smooth, stir in the Nutella until smooth. Transfer to a shallow bowl and dip the bottoms of each biscotti into the chocolate. Once dipped, place on a wax-paper lined cookie sheet until chocolate is set. Biscotti will keep in an airtight container for up to 2 months.