In a large bowl beat butter and both sugars until light and fluffy. Slowly add egg yolk, vanilla, and salt. Beat on low until well combined.
Add the flour to the butter mixture and combine.. The batter will be somewhat sticky.
Scoop the dough into portions of about 1 ½ tablespoons each and arrange on the baking sheet lined with parchment paper. Roll into balls and dip in sugar.
Press down the center of each ball with the back of a teaspoon, making a deep well in the middle. Fill with jam.
Bake for 10-15 minutes. Remove from the oven and allow to cool on the baking sheet for at least 10 minutes or until firm.
Sprinkle with confectioner’s sugar if desired.