Make the Curd: Whisk together eggs, sugar, cornstarch, salt, zest (if desired), lemon or lime juice, and vanilla extract in a bowl until smooth. In a saucepan, melt unsalted butter over medium-low heat, then pour in the egg mixture. Continuously stir for 5 to 10 minutes until thickened. Strain through a sieve and chill for at least 2 hours.

Prepare the Cookie Dough: Cream together softened butter and granulated sugar until light and fluffy, about 3-5 minutes. Beat in the egg and vanilla. Grind freeze-dried fruit into a fine powder and mix it with flour and salt before folding into the creamed mixture to form dough.

Shape the Cookies: Preheat your oven to 350°F (175°C). Roll dough into balls, about 1 inch in diameter, and use your thumb to create wells in each ball. Chill the cookie shapes in the refrigerator for at least 30 minutes.

Bake the Cookies: Bake cookies for 12 minutes, or until edges are golden. Remove and let cool on the baking sheet for a few minutes before transferring to a wire rack.

Fill with Curd or Jam: Once cooled, fill the indents with chilled curd or your chosen jam. Allow to sit briefly for flavors to meld before serving.