Preheat oven to 325°F. Line one baking sheet with parchment paper. Set aside.
In a large bowl, using an electric mixer, combine flour, cocoa powder, sugar, baking powder, and salt. Add the eggs, egg yolks, melted butter, and vanilla to the mixture. Beat until a dough starts to form. Add hazelnuts and mix until combined.
Remove dough from the bowl and divide into 2. Roll each portion of the dough into a log, 13-inch long. Place the rolls on the prepared baking sheet about 3 inches apart. Flatten the rolls slightly with your hands to about 1 1/2-inches wide.
Bake for 25 minutes. Remove from the oven and allow the rolls to cool down for 15 minutes. Transfer rolls to a cutting board. Using a sharp serrated knife, cut the rolls diagonally into ½-inch biscotti. Place the biscotti, cut side down, in the baking sheet and bake for 10 minutes. Remove pan from the oven, turn the biscotti over, and bake for 10 more minutes. Remove from oven and allow cooling over a wire rack.
Drizzle or dip biscotti with white chocolate. Allow the biscotti to stand until the chocolate sets. Serve.
Biscotti are traditional Italian cookies with a crunchy and crispy texture. Serve with a cup of espresso or cappuccino for dunking.