Whisk the 2 teaspoons egg white in a bowl to break it up. Add the pistachio nuts and coat thinly in the egg white. Sprinkle in the vanilla sugar, turning the nuts to coat, then spread out on a baking sheet lined with baking parchment. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 10 minutes. Leave to cool, then roughly chop.
Beat together the butter, 75 g (3 oz) of the caster sugar and the lemon rind in a bowl until very pale and fluffy. Beat in the egg yolks. Sift in the flour and baking powder, then stir in the nuts, reserving 3 tablespoons.
Whisk the egg whites in a clean bowl with a hand-held electric whisk until peaking. Gradually whisk in the remaining sugar, a spoonful at a time. Stir a third of the mixture into the creamed mixture using a large metal spoon. Gently stir in the remaining egg whites.
Spoon the mixture into a greased and lined 1 kg (2 lb) or 1.3 litre (2¼ pint) loaf tin (see