COMBINE FLOUR AND BUTTER: Place the flour and butter in the bowl of a food processor with the blade attachment fitted. Run the food processor until the mixture resembles fine bread crumbs.

MAKE THE DOUGH: Pour the flour and butter into the bowl of a stand mixer with the paddle attachment fitted. Add the powdered sugar, vanilla sugar and salt and mix to combine. Add the beaten egg and mix until the mixture begins to clump together.

FLAVOUR THE DOUGH: Divide the dough into two bowls - at this point, it should still be fairly loose and pebbly. Add two tablespoons of cocoa to one bowl and two tablespoons of flour to the other. Knead each dough briefly, just enough to bring it together. Pat each dough into a flat rectangle, wrap in clingfilm and chill for 45 minutes.

ROLL OUT THE DOUGH: Starting with the vanilla dough, roll it out on a lightly floured surface to a rectangle ½ a centimetre thick. Set aside and repeat with the chocolate dough.

TRIM THE DOUGH: Place the vanilla dough rectangle on top of the chocolate dough, turn so that one of the long sides is facing toward you and trim the edges. For a tidy finish, trim the vanilla dough ½ centimetre shorter on the long side furthest away from you. Squeeze together any dough cutoffs and re-roll and cut as desired.

SHAPE THE COOKIES: Fold back the top dough and brush lightly with water, pressing down gently to seal. Brush the top dough with water, then starting with the long edge closest to you, roll the dough gently, but tightly into a log, brushing the edge with water and pressing gently to seal. Transfer to the fridge and chill for 1 hour.

SLICE AND BAKE THE COOKIES: When the dough has chilled, heat the oven to 180°C / 350°F / Gas 4 and line an oven tray with baking paper. Use a sharp knife to cut the dough into 1cm rounds. Arrange the cookies on the oven tray allowing a little room to spread, then bake in the preheated oven for 12-14 minutes until lightly browned. Allow to cool on the tray for 5 minutes before transferring to a rack to cool completely.