Mix the room temperature butter with the powdered sugar until it becomes airy and creamy.
Separately mix the flour with the starch, cocoa, and salt.
Add the egg to the butter cream, and mix well until completely incorporated.
Gradually add the flour and mix on a low speed.
Finally, add the vanilla extract.
If the mixture is too stiff, add 2 tablespoons of milk.
Preheat the oven to 180 degrees Celsius (356 Fahrenheit) and line a large baking tray with baking paper.
Cut the dough into rounds with a pastry cutter and place each one on the tray, leaving 2-3 cm (0.8-1.2 inch) between each cookie.
Bake for 10-12 minutes, until edges are lightly browned.
Remove the tray from the oven and let rest for 2-3 minutes. Transfer them to a cooling rack.
Once completely cooled, decorate with melted dark chocolate and sugar shapes.
Enjoy!