In a medium bowl, whisk together egg, sugar, cornstarch, salt, citrus juice, and vanilla until smooth. In a saucepan over medium heat, melt butter and stir in the egg mixture. Cook for 5–7 minutes, stirring constantly, until thickened. Remove from heat and let it cool.

In a large mixing bowl, cream together softened butter and granulated sugar until light and fluffy, about 3–5 minutes. Add egg and vanilla extract, mix until combined. In a separate bowl, grind freeze-dried fruit, combine with flour and salt, and gradually incorporate into the wet mixture, ensuring uniform dough.

Divide dough into equal portions and roll into balls, about 1 inch in diameter. Create an indent in the center of each ball with your thumb. Place on a baking sheet lined with parchment paper and chill in the refrigerator for at least 30 minutes.

Preheat oven to 350°F (175°C). Bake chilled cookie dough for about 12 minutes or until edges are lightly golden and tops have set. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Once cookies have cooled, fill thumbprint indents with lemon or lime curd, or your favorite jam. This adds flavor and a pop of color.