Make the Lemon Curd:

1. In a small saucepan, whisk together the eggs, sugar, and lemon juice. Cook over medium heat, stirring constantly, until the mixture thickens (about 5–7 minutes).
2. Remove from heat and stir in butter and lemon zest until smooth. Transfer to a bowl, cover with plastic wrap (pressing it directly onto the surface), and refrigerate until chilled.

Make the Cookies:

1. Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
2. In a large bowl, beat the butter and sugar together until creamy. Add the egg yolks and vanilla extract, mixing until combined.
3. Gradually add the flour and salt, mixing until a dough forms.
4. Roll dough into 1-inch balls and place them 2 inches apart on the prepared baking sheet.
5. Use your thumb or the back of a spoon to make an indentation in the center of each ball.
6. Bake for 10–12 minutes or until edges are lightly golden. Remove from the oven and allow to cool slightly.

Assemble the Cookies:

1. Once the cookies are cooled, fill each indentation with chilled lemon curd using a spoon or piping bag.
2. Optional: Dust with powdered sugar for a decorative finish.