Melt the butter over a low heat and set aside until ready to use. Lightly beat the egg whites. Blend the flour and icing sugar in a food processor, add the chopped pistachios, egg whites, vanilla extract and then the melted butter. Continue to process until all the ingredients are fully incorporated and form a paste. Transfer the pistachio tuille mix to a clean bowl, cover with clingfilm and refrigerate for 2 hours to allow the mix to firm up before using.
Preheat the oven to 170°C. Place teaspoonfuls of the mix onto a baking tray lined with parchment paper. Leave a 6 cm. gap between each tuille as they will spread during cooking. Smooth out the tuille mix with a palette knife dipped in cold water. (Use a stencil to get evensized and shaped tuilles or a pastry cutter to cut them into shape as they come out of the oven.) Bake the tuilles for 6–7 minutes until lightly golden. Remove from the oven and allow the tuilles to cool for 1 minute. Lift them off the tray with a palette knife, place onto a wire rack or, drape them over a rolling pin to create curved tuilles and allow to cool and become crisp. Store the tuilles in an airtight container. If not using all the mix immediately, tightly cover with clingfilm and refrigerate. Beat the pistachio tuille mix before using again.