¾ cup caster sugar
Finely grated zest of 2 lemons
325g unsalted butter
2 tsp natural vanilla extract
450g plain flour
Food colouring (6 colours work best)*
Preheat oven to 180 degrees C. Place two eggs in a saucepan of cold water, bring to a simmer, then cook for 5 minutes. Cool in cold water, peeled, then separate the yolks, discarding the whites and press yolks through a fine sieve into the bowl of a stand mixer.
Add the sugar and lemon zest to the yolks and mix well, then add the butter and beat until extremely light. Beat in the vanilla, then fold in the flour to make a dough.
Divide into 6 equal pieces and tint each a distinct colour. Roll each piece of dough out to a 20cm x 15cm rectangle that is 4mm thick. Separate the final egg and whisk the white, discarding the yolk.
Brush a layer of dough with egg white, then repeat with the remaining colours. Divide the dough lengthways, then stack up. There should be 12 layers of colour. Refrigerate until firm.
Slice into 4mm thick biscuits, then arrange on a lined biscuit tray and bake for 15-17 minutes, until just firm.