Requires: Food processor (alternatively, use electric beaters), 2 shallow baking trays and a rolling pin
Preheat the oven to 180°C conventional oven (350° Fahrenheit/Gas 4). Adjust the metal racks to the middle groove of the oven. Prepare 2 sheets of non-stick baking paper onto a work surface. Line 2 baking trays with baking paper.
Using a food processor or electric mixer, beat the butter, castor sugar, eggs, vanilla bean and a pinch of salt until combined. Transfer the butter and sugar mixture to a large bowl. Set aside 30g of the flour. Stir 400g of the remaining flour into the mixture, tablespoon by tablespoon until just combined. As the dough forms, you will need to use your hands. Transfer to a clean work bench and knead until smooth.
Note: 00 is a powder-fine Italian type of flour. It is made from soft wheat varieties, and is frequently used in Italian desserts and pastas. 00 flour is available in most supermarkets, Italian food stores and gourmet delicatessens.
Divide the dough into 2 portions. 1 portion will make vanilla shortbread layer and the other portion cacao layer.
For the vanilla shortbread, add the remaining 00 flour (30g) and the lemon zest. Knead until well combined and smooth.
For the cacao shortbread biscuit dough, add the unsweetened cocoa powder (cacao magro). Knead until well combined and smooth.
Cover both biscuit doughs with separate sheets of plastic wrap and refrigerate for minimum 30 minutes.
Remove the dough from the fridge. Roll out each ball of dough onto the 2 prepared sheets of baking paper to get 2 rectangles about 7 mm thick and 22 X 27cm in size. Brush the entire vanilla shortbread rectangle with water. Transfer the cocoa shortbread rectangle directly on top of the vanilla shortbread. Gently press the outer edges and then use a rolling pin to lightly roll the cocoa layer. This will make the rectangle slightly larger 25 X 30cm approximately and bind the 2 pastry layers together. Brush the entire top cacao shortbread layer with water. Use the baking paper to roll up the shortbread from the short end. Wrap in cling firm and refrigerate for a minimum of 1 hour to firm up.
Remove the rolled shortbread from the fridge. Discard cling film. Cut discs about 7mm in thickness. Transfer the Girelle (Chocolate and vanilla swirl shortbread biscuits) to the prepared baking tray. Bake, one tray at a time in the oven for 10-12 minutes or until just golden. Transfer to a wire rack to cool completely – buon appetito.
Tip: Store any leftover girelle biscuits in an airtight container for up to 1 week.