Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
- In a mixing bowl, whisk together flour, cornstarch, sugar, salt, and lemon zest.
- Cut in the butter until crumbly, then mix in egg yolk and vanilla until a soft dough forms.
- Roll dough into 1-tablespoon-sized balls and coat lightly in granulated sugar if desired.
- Place on prepared baking sheet and use your thumb or the back of a teaspoon to make an indentation in the center of each ball.
- Bake for 10–14 minutes, until edges are just starting to turn golden.
- While still warm, fill each indentation with lemon curd.
- Cool completely before serving or storing.