In a medium saucepan, combine fresh lemon juice, zest, egg yolks, granulated sugar, and a pinch of salt. Whisk these ingredients together and place over medium heat, stirring constantly for 20-25 minutes until the mixture thickens and coats the back of a spoon. Once thickened, remove from heat and whisk in cold cubed butter until fully incorporated and smooth. Allow the lemon curd to cool completely before using.

In a large mixing bowl, whisk together all-purpose flour, baking powder, and sea salt until evenly combined. In a separate bowl, cream softened butter with granulated sugar until light and fluffy. Add in the egg yolk, vanilla extract, and lemon zest, mixing well. Gradually fold the dry ingredients into the wet mixture until just combined. Cover the dough and refrigerate for 1 hour to firm it up.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop tablespoons of chilled cookie dough and roll them into balls. Place them on the prepared baking sheet, leaving space between each. Use your thumb to create an indentation in the center of each cookie. Bake for 9-10 minutes, or until the edges are lightly golden, turning the tray halfway through for even baking.

Once your cookies have cooled on the baking sheet for a few minutes, transfer them to a wire rack to cool completely. Using a small spoon or piping bag, fill the indentation in each cookie with the prepared lemon curd. For an extra touch, dust the tops with powdered sugar right before serving to add a sweet finishing touch.