To Prepare

1. Preheat your oven to 170°C (325°F).  Line a baking sheet pan with parchment paper. 
2. Well mix the salt with the flour in a mixing bowl.  Set aside.  

To Make the Dough

1. In a mixing bowl, combine the butter and sugar. Mix by hand initially until combined.  Then beat the mixture with a handheld mixer at medium speed until light and fluffy. 
2. Add the egg yolk and vanilla extract.  Beat until they are incorporated.
3. Add in the flour mixture.  Beat until just combined.  
4. Weigh the dough and divide into two equal portions. (Total weight 632 g, 316 g each)
5. Add cocoa powder into one portion.  Mix until the cocoa powder is fully mixed using a spatula. Mix by hand to prevent over-mixing. 

To Roll the Dough

1. Sandwich the vanilla dough between two sheets of parchment.  Roll until it is 22 x 22 cm, 6 mm thick. 
2. Repeat the same process for the cocoa dough.  Try your very best to roll out both pieces into the same size and shape. They must have the same thickness.
3. Refrigerate the dough for at least an hour until firm.

To Assemble the Dough

1. Brush a thin layer of egg white on top of the sheet of vanilla dough and then stack the cocoa dough on top, align them as neatly as you can.
2. Cut the stacked dough in half with a sharp knife (need to measure this). 
3. Brush a thin layer of egg white on top of one half.  Carefully place the other half on top to create more layers.  Feel free to chill the dough at any point if the dough starts to soften.
4. Now you have a 4-layer dough stack in alternating colours. 
5. Next, cut strips from your dough stack to a 6 mm thickness which will create perfect squares (6mm by 6mm) for the pattern.  You should get 12 strips from the dough stack. 
6. Chill the strips in the fridge for at least 30 mins or until firm.
7. Brush a thin layer of egg white on one strip.  Stack another strip on top with the opposite colours to start creating the checkerboard pattern. 
8. Repeat the same process (Step 7), alternating colours, until you have a block of 4 layers, made up from 4 strips.  Set aside.
9. Now continue to build 2 more blocks of 4 layers by using the 8 remaining strips.
10. Place the three blocks on a lined baking pan and chill for an hour for easy slicing. 

To Bake the Cookies

1. To ensure even slices, use a ruler and cut each cookie about 6 mm thick as you work down the block. You should be able to cut 24 cookies per block, giving you a total of 72.
2. Place each slice on a lined baking pan with a little space (they shouldn’t spread much at all).   
3. Bake the sliced cookies in the preheated oven at 170°C (325°F) for 12 – 15 mins or until the edges begin to slightly brown. 
4. Let the cookies sit on the pan for 5 minutes and then transfer them onto a wire rack to cool completely.
5. Enjoy with a cup of coffee!