Preheat the oven to 350 °F (175 °C) and line baking sheets with parchment paper.
Cream together softened butter and granulated sugar in a bowl until light and fluffy.
Mix in the egg yolk, vanilla extract, and lemon zest until well combined.
Add the all-purpose flour and salt, mixing just until the dough comes together.
Chill the dough for 20 to 30 minutes to make shaping easier.
Roll dough into 1-inch balls and place them about 2 inches apart on the prepared baking sheets.
Press an indentation into the center of each dough ball using your thumb or the back of a spoon.
Fill each indentation with a small spoonful of lemon curd.
Bake cookies for 12 to 14 minutes, until edges are lightly golden but centers remain pale.
Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Dust with powdered sugar if desired before serving.