Preheat oven and toast almonds. Preheat oven to 180°C (350°F). Spread the whole almonds onto a baking tray and toast in the oven for 10 minutes. Allow to cool then chop coarsely. Keep the oven on. Line a baking tray with parchment paper.
Release oils from the orange zest. In a large mixing bowl add the sugar and orange zest and using your fingertips, rub the two together for 1 minute.
Add dry and then wet ingredients and mix. Into the bowl with the sugar, sift in the dry ingredients; flour, baking powder and salt.Make a well in the centre then add the wet ingredients; eggs, butter and almond extract. Using a wooden spoon, work from the middle out to mix the ingredients until just combined. Add the almonds and mix them into the dough. Add the chopped almonds and using the spoon or your hands, mix the almonds until combined and then gently knead the dough a couple of times until it comes together. The mixture will feel a little sticky and if you prefer you can refrigerate it at this point for 30-45 minutes to make it easier to handle when shaping. Shape dough. Lightly dust a work surface and your hands with flour. Divide the dough into two and using your floured hands, roll each portion into the shape of a log.Place each log onto a baking tray and continue to shape the two halves into a 30cm long x 5cm wide (12-in x 2–in) log. Leave space between the logs, as the dough will spread while baking. Use moistened hands to flatten the tops and smooth the dough.
Egg wash and sprinkle with sugar. Make an egg wash by whisking together the remaining egg and milk in a small bowl. Brush the egg wash over the logs, and then sprinkle them with the extra teaspoon of granulated sugar.
Bake and slice. Bake the biscotti for about 20 to 25 minutes, or until firm and golden. Remove from the oven and set aside to cool for 10 minutes. Reduce the heat to 135°C (275°F). After 10 minutes, using a sharp knife, cut the logs diagonally into 2cm (3⁄4-inch) slices. Place the slices on their sides on the parchment-covered baking tray.
Bake again. Bake the biscotti for another 15 to 20 minutes, or until dry and crisp, turning halfway through the baking time. Allow to cool fully. Once cooled, there will still be a little give in the biscotti. If you want very dry, hard biscotti, add another 10 or so minutes to the bake time.
Optional white chocolate decoration. Melt the white chocolate in a bowl over a pan of simmering water, ensuring the bottom of the bowl does not come into contact with the water. Once melted, remove from the heat and dip the end or drizzle chocolate over each biscotti before placing them back onto the baking sheet. Sprinkle finely chopped almonds over the white chocolate. Place the baking sheet into the fridge for 10 minutes to help the white chocolate set.