Combine Ingredients: In a small saucepan, whisk together caster sugar, cornstarch, and the egg until smooth. Add the fresh lemon juice and lemon peel.

Cook the Curd: Place the saucepan over medium heat, stirring constantly until the mixture thickens (about 3-4 minutes). Once thickened, remove from heat and stir in the butter until fully melted and combined.

Chill the Filling: Transfer the lemon curd to a small bowl, cover, and refrigerate for at least 1 hour, or until firm enough to handle.

Cream Butter and Sugar: In a large mixing bowl, beat the room temperature butter and caster sugar until light and fluffy. Add the lemon zest and vanilla, mixing until well combined.

Add the Egg: Mix in the egg until smooth and fully incorporated.

Combine Dry Ingredients: In a separate bowl, whisk together the flour, salt, and baking soda. Slowly incorporate the dry ingredients into the wet mixture, stirring gently until a soft dough begins to come together.

Chill the Dough: Refrigerate the cookie dough for 20-30 minutes to make it easier to handle.

Form and Fill the Cookies: Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper. Scoop a tablespoon of cookie dough and flatten it slightly in your hand. Place a small teaspoon of chilled lemon curd in the center, then top with another flattened piece of dough, sealing the edges to enclose the curd. Roll gently into a ball and coat in granulated sugar.

Bake: Place the stuffed cookie dough balls onto the prepared baking sheet, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are set and the tops are slightly crackled.

Cool and Serve: Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. Enjoy the burst of lemon flavor in every bite!