Step 1

Beat butter, ¼ cup (55g) of the granulated sugar, icing sugar, vanilla and one-third of the lemon rind using the paddle attachment of an electric mixer until pale and creamy. Beat in egg yolks, one at a time until just incorporated. With the mixer on low speed, beat in combined sifted flours in three batches until mixture forms large clumps. Turn out onto a piece of plastic wrap, flatten to a 20cm square, enclose and refrigerate for 1 hour to firm.

Step 2

Combine remaining ¼ cup (55g) granulated sugar and lemon rind with salt in a small bowl.

Step 3

Roll out dough on a lightly floured piece of baking paper until 4mm-thick. Using a floured fluted pastry wheel (or knife), cut dough into approximate 5cm x 6cm rectangles. Place on two, large, baking-paper line oven trays. Using one of the egg whites, brush top of cookies lightly, then sprinkle with salt mixture; freeze for 20 minutes to firm.

Step 4

Meanwhile, preheat oven to 170°C (150°C fan-forced). Bake biscuits for 12 minutes or until a biscuit can be pushed gently without breaking. Scatter hot shortbread with remaining salt mixture and leave to cool on trays.