In the stand mixer bowl, beat the yolks and sugar together. Add the softened butter and work until homogeneous, before adding the flour and baking powder.
Form a cylinder with the dough, wrap in cling film then place in the fridge 90 min.
Preheat the oven to 340ºF (170°C). Cut the dough into slices a bit thicker than ½ in (1.5 cm) thick. Place each slice at the bottom of a muffin mold to keep them from spreading.
Bake for 20 min, keeping an eye on the cookies and taking them out of the oven once the cookies start browning.
Let cool before removing from the muffin pan.