The butter needs to be soft (room temperature) but not melted.
Mix the egg yolks and sugar by hand until the mixture becomes airy.
Add the vanilla extract and mix.
Add the butter, sifted flour and baking powder, and mix quickly (either with your hands or a wooden spoon).
Put the dough on the table and use a silicone pastry scraper to press the dough towards the table, little by little. Do this twice. This avoids remaining lumps of butter in the dough.
Place the dough in between two sheets of baking paper and roll the dough until 1cm thick.
Leave the dough flat on a baking tray in the freezer for 30min - 1h, or until it is firm enough to easily cut out cookies. You can alternatively leave it for a couple of hours in the fridge.
Preheat the oven at 180°c or 350°F.
Place the cookies in the cookie cutter on a baking paper-covered baking tray. If you cook them without the cookie cutters, make sure you leave some space between them as they will float out.
Bake for approx 15min or until golden brown. Leave them to cool down on a grid.