Pre-heat the oven to 175°C (350°F) and line a baking tin with grease-proof paper.
Beat the butter, vanilla sugar and sugar together until nice and fluffy. Add the flour and mix until a dough forms. Place the dough in the fridge for around 15 minutes to firm up a little.
Roll into small balls and press your thumb down in each one making a nice, fairly thick hole.
Bake for around 10 minutes, just until the edges begin to brown (they will look pale and be very soft to the touch).
Remove from the oven. You will probably find the holes have closed up a little when baking, so just use the back of a small spoon (or similar) to press the cookie down and make the holes larger again.
Leave to cool on the tray for around ten minutes and then transfer to a wire rack to cool completely.
Melt the chocolate and then drizzle over the cookies. Enjoy!