Mix together butter, sugar, vanilla, and the pinch of salt until creamy.
Add the egg yolk and mix again until well combined.
Add the baking powder to the flour and mix together before adding them to the first mixture.
Add the ground nuts of you are using them too, then knead the dough briefly.
If needed, place the dough in the fridge for to be able to work with it easier.
Divide the dough into 15 small ball dough, place them in the moulds and use the finger to slightly spread it.
Bake for 15 minutes in a 170C / 338F preheated oven.
As soon as you take the cookies out of the over and while they are still hot, use a cocktail muddler or the base of a shot glass to press the center of the cookies and make a whole in the center.
Allow to cool.