Melt butter and dark chocolate using a double boiler. Ensure the bowl with chocolate is smaller than the water bowl to avoid water contact.

Add sugar and salt to the melted chocolate and butter mixture. Mix until the temperature cools to lukewarm.

Whisk the egg and add it to the mixture, stirring until combined.

Sift cake flour and baking soda into the mixture. Combine into a dough.

Cover the dough with plastic wrap and refrigerate for 30 minutes.

Divide the dough into 12g portions and roll into balls.

Roll the dough balls in powdered sugar to coat them completely.

Place the coated dough balls on a baking sheet with enough space between them. Bake in a preheated oven at 180°C for 5 minutes.

Press the tops of the cookies gently with a flat tool to create cracks, then bake for another 7 minutes at 180°C.