Preheat oven to 325°F (163°C).

In a large bowl cream the butter and sugar.

Add the egg yolk, vanilla extract, and salt. Mix well.

Add flour and mix it well with the other ingredients until it's nice and smooth.

Make small balls with the dough and place them on a cookie sheet lined with parchment paper or a silicone mat. The balls should measure about 1 tbsp and you can use a small ice cream scoop to make them all the same size.

Slightly flatten the top of each ball with your fingers and then with your thumb, or the end of a large wooden spoon, make a small well in the center of each cookie.

Fill each well with about ½ tsp of jam.

Bake for 12-15 minutes or until they are slightly golden brown on the bottom.