Take the butter out of the fridge 1 hour beforehand. It must be soft for easy creaming.
In the bowl of a stand mixer fitted with the paddle attachment, cream the soft butter with the sifted powdered sugar until smooth and fluffy. You can also do this in a mixing bowl using a spatula or even by hand.
Add the egg yolk and vanilla extract, mixing well to incorporate. Then add the milk and continue mixing until the mixture is fully combined.
Stop the mixer and scrape down the sides of the bowl. For small batches, you can finish mixing with a spatula if needed.
Sift in the flour and baking powder, and mix until you have a homogeneous dough. Do not overwork it. You can check the dough’s texture in the tips above.
Form the dough into a log and divide it in half to make two balls.
Wrap each ball in plastic wrap and flatten slightly. Chill in the refrigerator for 20 minutes before rolling it out between two sheets of parchment paper. This prevents the dough from sticking and avoids having to flour your work surface.
Roll the dough out to 3 mm (1/8 inch) thick, then place it in the refrigerator for 20 minutes to firm it up. This makes it easier to cut and transfer the cookies without distorting them.
Next, cut out the shapes using the cookie cutter included in the kit. I used the Silikomart's Cookies Choc and Decora's CiocCookies cutters. Each set comes with a cookie cutter and a silicone mold to make the chocolate tops.
Place the cookies on a baking sheet lined with parchment paper or, preferably, on a Silpat mat. You can also use a perforated baking tray.
Return the sheet with the shaped cookies to the refrigerator for about 20 minutes. Gather the scraps into a ball and re-roll as before.
Preheat the oven to 165°C (325°F) conventional heat.
Bake for 20 to 25 minutes. Keep an eye on them toward the end so they don’t become too dark. Pay attention to the first batch, as baking times can vary between ovens.
Transfer the cookies to a wire rack and let them cool completely.