Checkerboard cookies

This makes very pretty cookies, with a checkerboard-like pattern. Usually, I will make only four squares, but you can go nuts and go up to 9 or 16!

Tools

soften the butter (ideally overnight)
separate the eggs, keep both the yolks and whites
beat the butter until creamy
add in the powdered sugar and salt, mix until combined
add the egg yolks, mix until uniform
separate the mixture in half
gradually add 150 g of the flour, knead until combined
if it gets too hard, continue kneading by hand
remove the dough from the mixer, and continue with the other half of the mixture
whisk the cocoa powder with the remaining 130 g of flour
gradually add the cocoa and flour to the mixture, knead until combined
if it gets too hard, continue kneading by hand
place each dough in a bag
roll out the bagged doughs to equal rectangles, about 1 cm thick
freeze both for 10 minutes until firm
whisk the egg whites to loosen them up (do not beat!)
preheat the oven to 175 °C
place both doughs on a work surface, brush egg white on both tops and flip them together (using the egg white as a glue)
trim the edges so you have nice, straight rectangles
cut into equal strips, make sure these are squared
use more egg whites to glue two strips together to make the checkerboard pattern
cut the bars into cookies of equal thickness
place the cookies on a baking tray lined with baking paper
bake for 12 to 15 minutes at 175 °C
transfer to a rack and allow to cool
store in airtight container, or freeze them