01 -
Whisk sugar and cornstarch in a small heatproof bowl. Whisk in the egg, lemon juice, and lemon zest. Set over a pan of gently simmering water and cook, whisking often, until thickened, 7-9 minutes. Remove from heat and whisk in the butter.
02 -
Press plastic wrap directly on the curd and chill until cold. Scoop 12 small mounds onto a parchment-lined plate and freeze until firm, at least 1 hour.
03 -
Beat the softened butter, sugar, lemon zest, and vanilla until creamy. Beat in the egg. Add flour, baking soda, and salt, then mix just until a soft dough forms.
04 -
Cover and refrigerate the dough for 25-30 minutes, or until it is firm enough to handle without sticking heavily to your hands.
05 -
Heat the oven to 350°F (175°C) and line two baking sheets with parchment. Divide the dough into 12 portions. Flatten one portion, place a frozen curd mound in the center, and wrap the dough around it completely. Roll smooth and repeat.
06 -
Roll each stuffed dough ball in granulated sugar. Place 2 inches apart on the baking sheets and bake for 10-12 minutes, until puffed and just set at the edges.
07 -
Let the cookies cool on the baking sheet for 10 minutes, then move them to a rack. Cool at least 20 minutes more so the lemon center can settle.