Step 1: Make the Lemon Filling

1. In a small saucepan, whisk together the lemon juice, lemon zest, sugar, and egg yolks.
2. Cook over medium-low heat, stirring constantly, until the mixture thickens, about 5-7 minutes.
3. Remove from heat and stir in the butter until smooth.
4. Transfer to a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and refrigerate until cool.

Step 2: Prepare the Cookie Dough

5. In a bowl, whisk together the flour, baking powder, and salt.
6. In a separate bowl, cream the softened butter and sugar until light and fluffy.
7. Beat in the egg, vanilla extract, and lemon zest until well combined.
8. Gradually add the dry ingredients to the wet ingredients, mixing just until the dough comes together.
9. Divide the dough into two discs, wrap in plastic wrap, and chill in the refrigerator for 30 minutes.

Step 3: Shape and Bake the Cookies

10. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
11. Roll out one disc of dough to about ¼-inch thickness on a lightly floured surface.
12. Use a round cookie cutter to cut out shapes and place them on the prepared baking sheet.
13. Repeat with the second disc of dough. For half of the cookies, cut out a small hole in the center to create a top layer.
14. Bake for 8-10 minutes, or until the edges are lightly golden. Let the cookies cool completely.

Step 4: Assemble the Cookies

15. Spread a teaspoon of the chilled lemon filling onto the flat side of each full cookie.
16. Top with a cookie that has a hole cut out, pressing gently to sandwich the filling.
17. Dust with powdered sugar, if desired, for a decorative finish.