INGREDIENTSFor the thumbprints:
16 tbsp. unsalted butter
1 cup sugar
2 cups flour
1/2 tsp. kosher salt
For the lime curd:
1/3 cup sugar
1 tbsp. lime zest
3 egg yolks
1/2 cup fresh lime juice
1 tbsp. unsalted butter, cubed and chilled
INSTRUCTIONS1. Make the thumbprints: Heat butter in a small saucepan over medium heat until its solids begin to brown and smell nutty; pour into a small bowl, scraping bottom of pan for any browned bits, and refrigerate until solid. Place solidified brown butter in a bowl along with the sugar, and beat on high speed of a hand mixer until fluffy, about 3 minutes. Add flour and salt and beat on low speed until just combined. Using a l-oz. ice cream scoop or 2 tablespoons, portion and shape dough into 1" balls and place on parchment paper-lined baking sheets, spacing balls 2" apart. Using your index finger or the end of a wooden spoon, press into balls to create a deep well; refrigerate cookies until well-chilled, at least 1 hour.
2. Meanwhile, make the lime curd: Whish together sugar, zest, egg yolks, and egg in a 2-qt. saucepan until smooth; whisk in lime juice, and heat over medium heat. Cook, stirring constantly, until mixture thickens to the consistency of loose pudding; remove from heat, add butter, and whisk until smooth. Transfer to small bowl and press a piece of plastic wrap directly on the surface of the curd; refrigerate until well-chilled, at least 2 hours.
3. Heat oven to 375°. Bake cookies, rotating baking sheets top to bottom and front to back halfway through cooking, until dry to the touch and golden brown on the bottom, about 16 minutes; let cool completely. When ready to serve, fill cavities of each cookie with some of the lime curd and serve immediately.