Combine the flour, sugar, baking powder, salt and ground coffee in the bowl of an electric mixer.
In a separate bowl, whisk together the eggs, egg yolks and vanilla.
Add egg mixture to the dry ingredients. Using a paddle attachment at medium speed, mix the ingredients until almost combined.
Lower the speed to low and add the hazelnuts. Mix until nuts are evenly distributed into the dough. The dough will be soft.
Divide the dough into three pieces. Shape each into a log about 10 inches long and 2 inches wide. Place the logs several inches apart on a parchment-lined baking sheet, sprinkle with sugar if desired, and bake at 325 degrees until puffed and golden brown, about 20 minutes.
Cool the logs, then cut them on the diagonal with a serrated knife into ¾-inch thick slices.
Lower the oven heat to 300 degrees. Place the slices on baking sheets and bake until hard and dry, about 20 minutes.
Cool the slices. Melt the white chocolate in a small bowl set over a pot of simmering water.
Decorate the biscotti with the melted white chocolate as desired. When the chocolate has set, store the biscotti in a covered container in a cool, dry area.