Grease a baking sheet.
Put the butter, sugar and lemon rind in a bowl and beat together, then sift in the flour and baking powder. Add the egg yolk, egg white and pistachios and mix to a soft dough.
Divide the dough into 2 pieces and shape each roughly into a sausage measuring about 15 cm (6 inches) long. Space the 2 pieces well apart on the baking sheet and flatten each to a depth of 1 cm (½ inch).
Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 20 minutes until risen and turning pale golden. Remove from the oven and leave to cool for 10 minutes, leaving the oven on. Using a serrated knife, cut the biscuits across into 1 cm (½ inch) thick slices. Return to the baking sheet, cut sides face up and bake for a further 10 minutes to crisp up. Transfer to a wire rack to cool. Store in an airtight container for up to 1 week.