Line two baking sheets with parchment paper and set aside. Preheat oven to 170°C (340°F).

In a large bowl cream butter and powder sugar until smooth and creamy. Add egg yolk, vanilla extract and mix until combined. Add flour, salt and stir until incorporated and dough is formed.

Using a one-tablespoon cookie scoop or measuring tablespoon take 1 tablespoon of dough and roll into balls.

Then place the balls on prepared baking sheets. Slightly flatten the balls and then make an indentation with your thumb or with a back of a spoon into each ball. At this stage, if dough is too soft, refrigerate for 30 minutes. Then, fill each with cookie the jam.

Bake for 12-15 minutes, the cookies should still be pale, but slightly golden at the bottom.

Dust with powdered sugar (optional) and allow to cool on a wire rack.