Preheat oven to 350°F (175°C) and line a baking sheet with parchment.

Beat butter and sugar until light, about 2–3 minutes. Mix in egg yolk and vanilla.

Stir in flour and salt until a soft dough forms. Chill 15 minutes if too soft.

Roll dough into 1‑inch balls and place 2 inches apart on the sheet.

Press thumb into each ball to form a well; fill with a little jam.

Bake 10–12 minutes until edges are set but centers stay glossy. Cool on the pan 5 minutes, then transfer to a rack.