To make the dough, add the soft butter and sugar to a large mixing bowl, and use a hand mixer to cream them well.
Add the egg and mix again to get a mixture with a pale yellow colour and a smooth texture.
Sift in the flour, add the vanilla extract, and knead everything well into a dough that does not stick to the hands.
Cover the dough in clingfilm and refrigerate for 30 minutes.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit, 160 fan oven).
Dust the work surface with flour, and use a rolling pin to roll out the dough.
Use a larger cookie cutter to cut out the base of the cookie, then use a smaller cookie cutter to cut out the centre of the cookie, which will be the glass window.
Rework the dough until it's used up.
Line 3 baking trays with non-stick paper, and carefully transfer the cookies onto the trays.
Place a boiled sweet in the middle of the cookie.
Bake for 10 minutes, then remove the tray from the oven and leave the cookies to cool down completely onto the trays, the soft centre will set once they cool down.