Take the butter out of refrigerator and let it come to room temperature.
Smear a thin layer of butter on a baking sheet.
Preheat the oven to 350 degrees.
With an electric mixer, cream the butter for 2 minutes then add sugar and beat well until fluffy. If pale yellow, the butter will get lighter in color and the sugar will lose its graininess.
Add vanilla extract and mix well.
Add the flour and mix at low speed until incorporated.
Divide the dough into 24 balls. Place them at least 1 inch apart on the cookie sheet.
Using your fingers gently pat on the dough balls to flatten them out a bit.
Make a shallow well on each cookie with your thumb.
Roll your thumb back and forth to widen the well.
Fill the wells with about 1/4 teaspoon jam. Bake until very lightly browned around the edges, about 22 to 25 minutes.
The jam is especially very hot when taken out of the oven.
Cool the cookies completely before handling.