01 - Combine orange juice, lemon juice, lime juice, egg yolks, granulated sugar, and citrus zest in a small saucepan and whisk until smooth.
02 - Heat over medium-low, stirring constantly until thickened and mixture coats the back of a spoon, approximately 5 to 7 minutes. Remove from heat and whisk in cubed butter until fully incorporated. Transfer to a bowl, cover, and chill until set.
03 - In a large bowl, cream together softened butter and granulated sugar with an electric mixer until light and fluffy.
04 - Add egg yolk and vanilla extract; beat until smooth. Incorporate flour and salt and mix on low speed until just combined.
05 - Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper. Roll dough into 1-inch balls and space 2 inches apart on prepared sheets.
06 - Using your thumb or the back of a spoon, make an indentation in the center of each ball.
07 - Bake cookies for 10 minutes, then remove and gently reinforce the indentation if needed.
08 - Fill each indentation with 1/2 teaspoon citrus curd and return cookies to the oven for 5 minutes, baking until edges are barely golden.
09 - Allow cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely. Dust with powdered sugar before serving if desired.