In a bowl, knead the flour, starch and cocoa. Run the mixer using the dough hook, at low speed, until the ingredients are blended.
Add the cold butter cut into pieces and the egg. Let the mixer run until the dough becomes compact. Add milk to help with the blending and vanilla extract for flavor.
Add the baking powder and salt.
Transfer the dough to a board and knead it into a ball. Wrap it in plastic wrap, keep it in the fridge for at least half an hour, then roll out the dough using a rolling pin, into a sheet of 5 mm and use a wheel or a cutter to make squares that you will put on a baking tray lined with baking paper.
Bake at 180 °C (350F) for 15-20 minutes.
Once cooked, take them out and let them cool.
Decorate them with melted chocolate.