In a bowl, cream the butter and sugar together until combined then add the egg yolk, vanilla extract, and salt until smooth. Slowly add the flour and mix until no longer crumbly.
Roll into balls and place on baking sheet. Make an indentation into each ball using a teaspoon. Be careful not to press through. Chill for 1 hour in the fridge.
Spoon jam into indentations. Use room temperature jam. Bake at 350° for 10-12 minutes or until the cookies are slightly golden brown. Let them cool for 10 minutes.