Pour the sugar, pistachio powder and flour into a bowl. Mix well. Add the egg and melted butter. Beat the egg whites with an electric mixer. Add ⅓ of the egg whites to the batter and mix to soften. Add the remaining egg whites and mix gently. Pour onto a baking tray, smoothing the surface. Bake at 180°C for 10 to 12 minutes. Leave to cool, then cut out a 20×11 cm rectangle to fit your pastry frame.