Gently melt butter in a large bowl on Defrost in the microwave, about 30 seconds until soft. Add flour and ½ cup of the sugar. Stir with a wooden spoon until moistened and add yolk and vanilla. Continue stirring until dough begins to form.
Then, knead the dough with your hands until a firm ball forms.
Scoop and roll dough into small teaspoon-sized balls. Roll in between your palms until dough is a small smooth ball.
Pour the ⅓ cup of sugar on a plate and roll the balls to lightly coat. Place balls 2-inches apart on a parchment-lined baking sheet.
Using your thumb, the back of a spoon, or a tart tamper make an indention in the center of each ball.
In a small bowl, with a fork, whisk the jam until lumps are smooth. Fill each indention with ½ teaspoon of jam or preserves. (A very small spoon works well.) Place baking tray uncovered in freezer for 30 minutes. (Do not skip this step- dough needs to firm up to maintain shape during baking.)
Preheat Oven to 350 ℉.
Bake for 16 to 18 minutes, or until pale and just golden on edges. Leave on the pan until cool.
Optional* When completely cool, you may dust with powdered sugar for extra dazzle.