
In a medium bowl, mix 1/4 cup caster sugar and 1 tsp cornstarch until well combined.
Add 1 egg, 3 tbsp fresh lemon juice, and the zest from 1/4 lemon to the bowl, whisking until smooth.
Set up a bain-marie (double boiler) and pour the mixture into a heatproof bowl. Cook over simmering water for about 10 minutes, stirring continuously until thickened.
Remove from heat and stir in 1.5 tbsp unsalted butter until melted and smooth.
Transfer the lemon curd to a container and chill in the refrigerator until ready to use.
In a large mixing bowl, cream together 1/2 cup unsalted butter and 1/2 cup caster sugar until light and fluffy.
Add in the zest from 3/4 lemon and 1 tsp vanilla, mixing well.
Beat in 1 egg until well combined.
In a separate bowl, sift together 1 + 2/3 cup all-purpose flour, pinch of salt, and 1/2 tsp baking soda.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Cover the dough with plastic wrap and chill in the refrigerator for 20 minutes.
Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
Using a cookie scoop, take a ball of cookie dough and flatten it in the palm of your hand.
Place a frozen scoop of lemon curd in the center and fold the dough around it, rolling it back into a ball.
Roll the cookie ball in granulated sugar to coat.
Space the cookies on the prepared baking sheets and bake for 8-9 minutes, or until they are soft in the center.