In a large bowl, cream together softened butter and sugar until light and fluffy.
Mix in the egg yolk and vanilla extract.
Gradually add the flour and salt, mixing until just combined.
Roll dough into 1-inch balls and place them on a parchment-lined baking sheet.
Use your thumb or a small spoon to press an indent into the center of each cookie.
Spoon ½ tsp of jam into each indentation.
Preheat oven to 350°F (175°C).
Bake for 10-12 minutes until edges are lightly golden.
Let cool on the baking sheet for 5 minutes, then transfer to a wire rack.
Mix powdered sugar and milk until smooth, then drizzle over cooled cookies.
