Melt the butter and let it cool (to 38–40 degrees). Pour it into a mixing bowl and add raw cane sugar, demerara sugar, vanilla sugar, eggs, and egg yolks.
Whisk it together.
Add the sourdough starter and whisk.
Whisk until the mixture has lightened in color.
Measure the flour, baking soda, fleur de sel, and cocoa into a bowl.
Mix it together with a spoon.
Add half of it to the batter and whisk it in.
Add the other half of the flour and make sure it's thoroughly mixed in.
Chop the chocolate into large pieces and add it to the mixture.
Fold in the chocolate until it’s well distributed. Let the mixture rest for at least 1 hour at room temperature, or preferably 3 hours. If you have plenty of time, you can then refrigerate it until the next day (so the dough has matured/risen the old-fashioned way).
Roll the dough into 20-gram balls and place them on a piece of paper in the freezer until ready to bake.
Preheat the oven to about 165 degrees (not convection) and bake each sheet for about 6–7 minutes.
Let the cookies cool on a wire rack until they are completely cool, then place them in a jar or a cookie jar lined with parchment paper.