Preheat your oven to 350°F (175°C).
In a mixing bowl, cream together softened butter and sugar until light and fluffy.
Add egg yolk and vanilla extract to the mixture, mixing until combined.
Gradually add the flour and salt, mixing until just combined.
Scoop out tablespoon-sized amounts of dough and roll them into balls.
Place the balls on a baking sheet lined with parchment paper.
Use your thumb or the back of a spoon to create an indentation in the center of each ball.
Fill each indentation with approximately a teaspoon of your chosen jam.
Bake in the preheated oven for about 12-15 minutes or until the edges are lightly golden.
Remove from oven and allow to cool on the baking sheet for a few minutes before transferring to a wire rack.