Whisk lightly butter, add sugar, powdered sugar, baking soda, Fleur de sel (can be plain) and vanilla extract. Add the flour with cocoa and chocolate pieces, mix well, knead slightly just to combine ingredients. Divide the dough into 3 parts, each part forming into a sausage. Wrap in plastic wrap and place in the refrigerator for at least 2 hours. Cut into slices 1 cm thick. Put the slices on the baking tray lined with parchment paper. Bake for 10-12 minutes in a preheated oven 170 C (350 F). It is important not to overdo!!!