Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
In a large bowl, beat butter and powdered sugar until light and fluffy.
Add egg yolk and vanilla, and mix until smooth.
In another bowl, whisk together flour, salt, and cornstarch.
Gradually add dry ingredients to wet ingredients and mix into a soft dough.
Roll dough into 1-inch balls and place on baking sheets.
Use your thumb or the back of a spoon to make an indent in each cookie.
Fill each with ½ teaspoon of jam or jelly.
Bake 10–12 minutes or until edges are just set but not browned.
Cool completely, then dust with powdered sugar if desired.