In a heatproof medium size bowl, mix sugar and cornstarch.

Mix in the egg then the lemon juice and add the lemon peel.

Cook over bain marie, mixing often, for about 8 minutes or until starts thickening: then cook, mixing constantly, for another 2 minutes until thick.

Remove from heat, incorporate butter and mix to cool down quickly.

Cover with plastic wrap on the curd and put in refrigerator to chill for at least 30 minutes.

Spoon lemon curd onto a lined small tray diving equally into 10 spoons.

Put in freezer for at least 1 hour or until completely hard and frozen. You can prepare Lemon Curd Filling the day before.

In a large bowl mix butter, sugar, lemon zest and vanilla until smooth, creamy and well combined.

Mix in the egg using a whisk.

Sift in flour, salt, baking soda and gently mix with a spatula until well combined. You'll get a soft but not sticky dough.

Place the dough in refrigerator for at least 20 minutes or until easily handle, mostly if you live in a hot climate.

Scoop the dough using a large 5cm-2inch ice cream scooper, flatten and place one frozen Lemon Curd Filling scoop at the center. Then fold the cookie dough sides around the Lemon Curd Filling and roll to get a cookie ball.

Coat the cookie ball into granulated white sugar and place onto a lined baking tray at least at 5cm-2inch apart.

Repeat until finish all the cookie dough. You'll get 9-10 cookies. Work fast to avoid melting the Lemon Curd Filling.

Bake in preheated fan oven at 180°C-350°F for 8-9 minutes. Do not overbake, the center has to be very soft adn the sides just slightly golden.

Remove from the oven and cool down onto the baking tray.