Line a shallow baking tray with a silicone baking mat (or alternatively, use baking paper). Place 25g of pistachios onto a chopping board. Finely chop the pistachios, then transfer to a small bowl. Set aside the chopped pistachios.
Place the remaining 100g of the pistachios into a food processor. Process until the mixture resembles coarse breadcrumbs. Alternately, place the pistachios into a plastic bag and beat with a rolling pin until the mixture resembles coarse breadcrumbs. Add the icing (confectioners’) sugar, lemon zest and lemon extract to the pistachio crumb.
Use a whisk or fork to lightly beat the egg white until slightly foamy. Use your hands to work the egg white into the pistachio crumb mixture, a little at a time until it forms a firm paste (you will not need the whole egg white).
Roll one small tablespoonful of the dough (weighs approximately 15g) into a ball, then toss in the chopped pistachios to coat. Place the pistachio cookie onto the prepared baking tray. Repeat using the remaining cookie dough, ensuring that the balls of dough are well spaced out to allow room for spreading.
Use your finger to lightly press in the centre of each ball to slightly flatten. Refrigerate for a minimum of 30 minutes.
Meanwhile preheat the oven to 180°C conventional oven (356° Fahrenheit/Gas 4).
Remove the tray with pistachio cookies from the fridge. Bake on the middle groove of the oven for 10 minutes. Transfer to a wire rack to cool completely before serving – Buon appetito!
Note: The Gluten-free pistachio cookies will be a little soft to the touch when removing from the oven and will firm up once cooled.