Place the butter, salt and icing sugar (powdered sugar) into a large bowl.

Cream the ingredients together until the mixture is light and fluffy. If you are using an electric stand mixer, use the flat-paddle attachment.

Add the egg yolks.

Beat to combine well.

Weigh the mixture in a clean bowl.

Halve the mixture into separate mixing bowls.

Place one half of the Cookie Dough Base into the mixing bowl.

Add the additional ingredients for Vanilla Cookie Dough.

Beat the mixture together until the mixture resembles wet sand.

Add just enough water to bring the mixture together into a soft dough.

Remove the mixture to the kitchen bench.

Knead the dough lightly so that all of the ingredients are well-combined and there are no air-pockets in the dough.

Roll the dough between two sheets of baking paper until you have a rectangular shape measuring about 20 x 30 cm (8 x 12 inches), and about 5 mm thick. To achieve this shape, cut off excess strips of pastry and join them where necessary to build a rectangle. 

Place the Vanilla Cookie Dough to chill in the fridge while you prepare the Chocolate Cookie Dough.

Repeat the same steps as for the Vanilla Cookie Dough.

The Chocolate Cookie Dough will likely need more water as the cocoa powder will make the dough more dry.

Place the Chocolate Cookie Dough in the fridge to chill for about 15 minutes.

Remove both pieces of dough from the fridge.

Place the Chocolate Cookie Dough on top of the Vanilla Cookie Dough. Keep the baking paper underneath the Vanilla Cookie Dough.

The next step requires the dough to be soft and pliable for rolling, so you will need to allow the dough to come to room temperature before continuing. If you try to roll the dough when it is too cold, it will be difficult to roll and it may also crack.

Once the dough is soft and easy to handle, use the baking paper underneath the Vanilla Cookie Dough to lift up both pieces of dough. 

Start rolling the cookie dough into a tight log. Use the baking paper to firmly roll the dough, making sure there are no gaps in the layers.

Once all of the dough has been rolled into a log, gently press the edges into the side of log so that you have a nice, round log.

Roll the cookie dough log in the baking paper.

Place the cookie dough log in the fridge for 30-60 minutes to chill.

Place a sheet of baking paper on a large baking tray.

If you want even-sized cookies, use a ruler to mark the width of each cookie (about 1 cm thick). Otherwise, you can eye-ball it and cut as you please.

Use a sharp knife to slice the cookie dough.

Arrange the cookie slices on the baking tray. The cookies do not spread very much upon baking, but space them apart to allow them to bake evenly.

Place the tray of cookies into the fridge to chill for about 30 minutes.

Preheat the oven to 180°C/356°F (without fan).

Preheat a metal tray on the middle shelf in the oven.

Place the tray of cookies on the preheated metal tray.

Bake the cookies for 15-18 minutes, or until the cookies are lightly golden.

Remove the tray from the oven.

Leave the cookies on the hot tray for about 5-10 minutes to firm up.

Remove the cookies to a wire rack to cool completely.