Sift the flour and powdered sugar together until smooth. Then mix in the cocoa powder, salt, and baking soda.

Let the butter soften at room temperature, then beat it with the egg yolk and vanilla extract until smooth.

Add the flour mixture to the butter and egg mixture, and knead until it forms a dough. Mix in 2/3 of the chocolate chips.

Shape the dough into small balls and flatten them. Sprinkle the remaining chocolate chips on top.

Preheat the oven for 5 minutes.

Bake the cookies at 350°F (180°C) for 15 minutes. When you take them out, they will be slightly soft. Do not touch them and let them cool completely to harden.