Preheat the oven to 325 degrees F.
Use a stand mixer to beat the softened butter. Add in the powdered sugar, egg yolk and vanilla and mix well. Slowly beat in the baking flour until a dough forms.
On a baking sheet lined with parchment paper, roll out small balls of dough and use your finger to gently press an indentation in the center of the cookie (forming a small crater).
Fill each crater with 1/2 teaspoon of the jam of your choice. If you want to make chocolate ones, bake with the center empty and then add in melted chocolate as soon as the cookies come out of the oven. Let the cookies cool and then enjoy!