Lemon Curd Filling Preparation

In a medium-sized, heatproof bowl, combine the caster sugar and cornstarch.
Whisk in the egg, lemon juice, and lemon peel.
Set the bowl over a bain-marie, stirring often, and cook for about 8 minutes until the mixture starts to thicken. Continue stirring for an additional 2 minutes until thick.
Remove from heat, stir in the butter, and mix to cool the mixture quickly.
Cover directly with plastic wrap and refrigerate for at least 30 minutes (can be made a day ahead).
Divide the chilled lemon curd into 10 portions on a lined tray and freeze until solid (at least 1 hour).
Lemon Cookie Dough Preparation

Cream together the butter, sugar, lemon zest, and vanilla until smooth.
Beat in the egg.
Sift in the flour, salt, and baking soda, folding until the dough is soft and non-sticky.
Refrigerate the dough for at least 20 minutes.
Flatten scoops of dough, place a portion of frozen lemon curd in the center, fold, and roll into a ball.
Coat each ball in sugar and place on a lined tray, spaced apart.
Bake at 180°C (350°F) for 8-9 minutes until the edges are slightly golden.
Allow to cool on the tray.