Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream the softened butter and powdered sugar together until pale and fluffy.
Beat in the egg yolk, vanilla extract, and fresh lemon zest until fully incorporated.
In a separate bowl, whisk together the flour and salt.
Gradually fold the dry flour mixture into the butter mixture until a dough forms.
Shape the dough into small, 1-inch balls and place them on the prepared baking sheet.
Use your thumb or the back of a measuring spoon to make an indentation in the center of each ball.
Bake for 10-12 minutes until the edges are just barely golden.
Remove from the oven, gently re-press the center indentations if needed, and let cool for 5 minutes.
Fill each center with a small dollop of lemon curd.