In the bowl of a stand mixer with a flat beater: mix the sugar and butter.

Add the flour and salt, then mix again. You should get a stiff, compact dough.

Divide the dough into two equal balls (use a scale). I got about 410 g per ball.

Place one half in the bowl of the stand mixer, add the cocoa powder, then mix.

Shape rectangles about 2 x 2 x 6 inches (5 x 5 x 15 cm) each. Wrap each rectangle individually in plastic wrap, then place them in the refrigerator for at least 2 hours and up to 3 days for a long fermentation.

Cut each rectangle into 9 sub-rectangles: you get 9 yellow rectangles and 9 brown rectangles.

Create rectangles with three sub-rectangles: one yellow - one brown - one yellow. And one brown - one yellow - one brown.

Use egg white or water to stick them together. Stack 3 such rectangles on top of each other to form a new block consisting of 9 rectangles.

Wrap tightly with cling film and place in the refrigerator for 1 hour.

Preheat the oven to 350°F or 175°C.

Cut slices about 2 cm thick. Place the slices on a baking sheet lined with parchment paper.

Bake for 15 to 20 minutes, until the edges turn golden.