Preheat the oven to 375 degrees.
Cream together the butter and sugar until light and fluffy.
Add in the egg yolk and vanilla extract.
Mix in the flour and cornstarch.
Separate the dough into 10 equal sized balls and place on a parchment lined baking sheet.
Using a large spoon or small measuring cup create an indentation in the middle of the cookie dough ball.
Use your hands to form a bigger indentation in the middle and make sure to press the sides up a bit, this will hold the curd after baking.
Bake the cookies for 12-15 minutes until slightly golden brown.
While the cookies are still warm use a spoon to press the indentation back in case it lost its shape any during baking.
To make the curd to a medium sauce pan add your sugar, lemon zest, lemon juice and 4 egg yolks.
Whisk over medium heat until completely mixed.
Whisk constantly together until the mixture reaches 170 degrees and has thickened up.
Slice up the butter.
Remove from the heat and stir in the butter until all melted and combined.
Set aside to cool and thicken more.
Once the cookies and curd are cool, spoon some curd into each cookie.
Set aside.
For your meringue you will add the sugar and egg whites in a double boiler over simmering water.
Whisk the egg whites and sugar together constantly until the mixture reaches 160 degrees.
Carefully pour the mixture into the bowl of your stand mixture and using the whisk attachment whip until stiff glossy peaks form.
Once you have your stiff peaks add the meringue to a piping bag or ziploc bag with the corner snipped off.
Pipe your meringue onto the cookies.
Using a kitchen torch carefully brown the meringue.
Serve!