Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.
Cream softened butter and granulated sugar until light and fluffy, about 2–3 minutes.
Beat in the egg yolk and vanilla extract until fully incorporated.
Whisk together flour and salt, then add gradually to the butter mixture until a soft dough forms.
Scoop tablespoon-sized portions of dough and roll into smooth balls; optionally roll in powdered sugar.
Place balls 2 inches apart on the prepared baking sheet.
Use your thumb or the back of a small spoon to press a shallow well in the center of each ball.
Chill the prepared cookie sheet in the fridge for 15–30 minutes to prevent spreading.
Bake for 10–12 minutes until edges are set and tops are pale golden.
Remove from oven and immediately press wells again if needed; cool 2 minutes then transfer to a rack.
Spoon about 1/2 teaspoon of jam into each well while cookies are slightly warm.
Let cookies cool completely so jam sets; store or serve once jam is no longer sticky.