Preheat your oven to 350°F and line a baking sheet with parchment paper or a silicone mat to prevent sticking.

Cream chilled unsalted butter and granulated sugar in a large bowl until pale, light, and fluffy, about 2 minutes on medium speed.

Add the egg yolk and vanilla extract, mixing gently until a smooth, cohesive dough forms and starts to pull away from the bowl sides.

Stir in flour and salt gradually, folding until just combined; be careful not to overmix to keep cookies tender and melt-in-your-mouth.

Shape dough into 1-inch balls, then roll between your palms until smooth.

Indent each ball gently with your thumb to create a small ½-inch well.

Spoon about ½ teaspoon of your favorite fruit jam into each well, filling to just below the edge.

Bake cookies for 12–14 minutes until edges are lightly golden brown while the centers remain soft and jam looks set.

Cool on the baking sheet for 5 minutes, then transfer cookies to a wire rack to cool completely before serving.

Dust lightly with powdered sugar for a pretty, festive finish.