Pre-heat the oven to 350 and line a sheet tray with a silicone mat. Whisk the flour, sugar, cinnamon and baking powder together in a mixing bowl, then stir in the hazelnuts and pistachios. In another bowl, whisk the eggs and vanilla together. Pour the wet ingredients into the dry ingredients and stir it together well until it becomes a soft and very sticky dough.
Turn the dough out onto a well floured and clean surface. Divide it in half, then with floured hands form each half into a log that goes almost the length of the sheet pan. Place the logs side by side on the sheet tray with plenty of space in between them, then just lightly press down on the log to make the side on the tray flat. Bake them for 30 minutes. When they are done, let them cool for at least 10 minutes or so on the trays. Leave the oven on.
When they have cooled, transfer the logs to a cutting board and slice each log into 1/2 inch thick biscotti. Lay them back out on the sheet tray with the cut side down and bake them for another 20 minutes. Let them cool again and serve with great espresso, coffee or tea! These will also keep in sealed containers for about a week.