Whisk the egg, granulated sugar, cornstarch, salt, and citrus zest in a small bowl until completely smooth. Stir in the freshly squeezed lemon or lime juice and vanilla extract.
Melt the butter in a small saucepan over low-medium heat to begin your vibrant citrus curd.
Pour the citrus mixture into the saucepan and whisk continuously over medium-low heat for 5 to 10 minutes. Remove from heat once the curd thickens, bubbles slightly, and heavily coats the back of a spoon.
Stir in a drop of green food coloring if you are making lime curd and desire a richer green color.
Strain the finished curd through a fine mesh sieve into a jar to guarantee a silky smooth texture. Cover tightly and chill in the refrigerator for at least 2 hours or overnight.
Line a baking sheet with parchment paper to prepare for baking the fruit filled cookies.
Cream the butter and granulated sugar in a stand mixer fitted with a paddle attachment until light and fluffy. Add the egg and vanilla extract, mixing until fully incorporated.
Blend the freeze-dried fruit in a food processor or blender until it forms a fine powder. Use a rolling pin or mortar and pestle to crush any remaining stubborn chunks.
Whisk the all-purpose flour, salt, and pulverized fruit powder together in a separate bowl. Gradually add these dry ingredients to the wet mixture, mixing until a soft, colorful dough forms.
Portion the dough using a cookie scoop and roll into 12 smooth balls. Place them onto the prepared baking sheet and gently press your thumb or the back of a measuring spoon into the center of each to create a distinct well.
Chill the prepared thumbprint cookies in the refrigerator for a minimum of 30 minutes to prevent the dough from spreading while baking.
Bake the cookies at 350ºF for 12 minutes. Allow the cookies to cool completely on the baking sheet before generously filling the centers with the chilled citrus curd or your favorite fruit jam.