In a medium bowl, combine 55 grams of caster sugar and 1 teaspoon of cornstarch, mixing well. Whisk in 1 large egg, 3 tablespoons of fresh lemon juice, and the peel from 1/4 lemon. Cook this mixture over a bain-marie for about 10 minutes, stirring regularly until it thickens. Once thickened, remove from heat, add 22 grams of unsalted butter, stir until melted, and let it cool. Transfer portions to the freezer for at least 1 hour.

In a large mixing bowl, cream together 110 grams of room temperature unsalted butter and 110 grams of caster sugar until the mixture is smooth and fluffy, about 4-5 minutes. Mix in the zest from 3/4 of a lemon and 1 teaspoon of vanilla extract, then add 1 large egg until fully combined. Sift in 200 grams of all-purpose flour, a pinch of salt, and 1/2 teaspoon of baking soda, folding gently until just incorporated. Chill the dough in the refrigerator for 20 minutes to help it set.

Preheat your oven to 180°C (350°F) and line a baking tray with parchment paper. Scoop a tablespoon of cookie dough and flatten it in your palm, then place a frozen portion of lemon curd filling in the center. Carefully fold the cookie dough around the filling and roll it into a ball. Roll each ball in granulated white sugar and place them on the prepared tray, ensuring they are spaced apart to allow for spreading.

Slide the baking tray into your preheated oven and bake the Stuffed Lemon Cookies for about 8-9 minutes. Keep an eye on them; they should be soft in the center with slightly golden edges. Once baked, remove the tray from the oven and allow the cookies to cool on the tray for a few minutes, as they will continue to set as they cool down slightly.

After letting the cookies cool slightly, transfer them to a wire rack to cool completely. These Stuffed Lemon Cookies can be served warm or at room temperature.