In a medium saucepan, whisk together the lemon juice, lemon zest, egg yolks, sugar, and salt. Cook over medium-low heat, stirring constantly for about 20-25 minutes until the mixture thickens.

Remove from heat and whisk in the cold butter until smooth. Transfer to a bowl and refrigerate until firm.

In a mixing bowl, whisk together the flour, baking powder, and sea salt.

Cream softened butter with granulated sugar until light and fluffy, about 3-4 minutes. Mix in an egg yolk, vanilla, and lemon zest, then fold in dry ingredients until just mixed. Chill the dough for about 1 hour.

Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper. Scoop dough into 1-inch balls and place on baking sheets, spacing 2 inches apart.

Create shallow wells in the center of each cookie and bake for 10 minutes until edges are golden. Avoid overbaking.

Remove cookies from oven and while still warm, fill centers with lemon curd using a teaspoon or piping bag. Dust with powdered sugar if desired and cool completely before serving.