In a heatproof bowl, combine the caster sugar and cornstarch, then whisk in the egg and fresh lemon juice until smooth. Place the bowl over a simmering pot of water and cook for about 8–10 minutes, stirring constantly until the mixture thickens and coats the back of a spoon. Remove from heat, mix in the butter until melted, and chill for 30 minutes.

Once the lemon curd is chilled, divide it into 10 equal portions and roll each into a ball. Place the lemon curd balls on a parchment-lined plate or tray and freeze for at least one hour.

In a large mixing bowl, cream the unsalted butter and granulated sugar together until light and fluffy, about 3–5 minutes. Add the lemon zest, vanilla paste, and egg, blending until combined. Then, sift in the all-purpose flour, salt, and baking soda, mixing just until incorporated. Chill the dough for 20 minutes.

Preheat your oven to 180°C (350°F) and line a baking sheet with parchment paper. Take about 1 tablespoon of chilled cookie dough and flatten it in your palm. Place a frozen lemon curd ball in the center, enclose the dough around it, and roll into a smooth ball. Repeat with all the dough and lemon curd.

Roll each cookie ball in granulated sugar and place them on the prepared baking sheet, spacing them apart. Bake for 8–9 minutes, until soft with lightly golden edges. Let cool on tray for a few minutes before transferring to a wire rack.