Preheat the oven to 180°C. Melt the chocolate and 20 grams of butter together. Mix the fine sugar and brown sugar with the rest of the soft butter and stir until creamy. Add the melted chocolate to the mixture. Stir well and smooth.
Sieve the flour and sprinkle it with the cornstarch to the chocolate mixture. Mix and finally add the egg to it. Work into a smooth dough. Put in the fridge for 30 minutes.
Roll out the dough and cut out biscuits with shapes. Place them far enough apart on a baking tray covered with baking paper.
Bake the cookies in the oven at 180°C for 8 to 10 minutes. Let them cool on a grid. Finish off with sieved cocoa powder if necessary.