Prepare the Lemon Curd: In a small saucepan, whisk together sugar and eggs until smooth. Add in fresh lemon juice and a pinch of salt, and heat over low. Stir continuously for about 10-15 minutes until thickened. Remove from heat, stir in butter until melted, then chill in the refrigerator for about 1 hour.

Make the Cookie Dough: Cream together softened butter and sugar until light and fluffy. Incorporate the egg and vanilla extract. Whisk together flour, baking powder, and salt, then gradually add to the wet ingredients. Wrap the dough and chill in the refrigerator for at least 1 hour.

Shape the Cookies: Preheat oven to 350°F. Roll out dough between parchment paper to 1/4 inch thickness. Use cookie cutters to shape cookies, cutting out centers for filling. Chill shaped cookies in the fridge for 15 minutes.

Bake: Arrange cookies on a baking sheet and bake for about 8-10 minutes until bottoms are golden brown. Allow to cool on the tray before transferring to a wire rack.

Assemble: Once cool, spread lemon curd on whole cookies and top with cut-out cookies to form sandwiches. Chill assembled cookies in the refrigerator for 30 minutes before serving.