Place the pistachios into a dry pan and toast over medium-high heat until smelling nutty. Remove from the pan and set aside to cool.
Place the sugar, honey and water into a large saucepan. Place over medium heat and wait for the sugar to dissolve. Do not stir.
Place the egg whites into the metal bowl of a stand mixer, fitted with the whisk attachment. Add the green food colouring, if using (the colour will pale significantly when the nougat is whipped, but you can always add more colouring later on if needed). Set aside for now.
Once the sugar has dissolved, keep the syrup over the heat and allow to bubble away. Monitor the temperature using a candy thermometer.
Once the syrup reaches about 130°C, start whipping the egg whites on high speed until they reach stiff peaks.
Once the syrup reaches 140°C, remove it from the heat and, with the mixer running on high, pour it into the egg whites in a slow, steady stream down the side of the bowl.
Turn the mixer speed up to full and leave the nougat to whip for about 15 minutes, until the mixture is thick, stiff and beginning to tear. When you stop the machine and lift the whisk, the ribbon falling from it should tear off. The nougat will still be warm (as will the metal bowl, so be careful!).
At this point, work quickly, because the nougat will get harder to work with the cooler it gets. Add the toasted pistachios and use a wooden spoon to stir them into the nougat until they are evenly distributed.
Place one of the wafer paper sheets onto a clean work surface. Tip the nougat onto the wafer paper. Use a lightly oiled spatula to flatten out the nougat slightly, then press the second wafer paper sheet on top of it. Use a rolling pin to gently and evenly flatten out the nougat until it is your desired thickness (mine was approx. 2.5cm/1 inch).
Leave the nougat at room temperature to cool for a couple of hours, then transfer the slab to an airtight container and leave overnight to set. Once set, trim off any excess wafer paper around the edges with a pair of scissors, then use a sharp, lightly oiled knife to cut it into squares.
Store the nougat in an airtight container at room temperature. It will keep for about 1 month.