In a small bowl whisk together the sugar and cornstarch. Set aside.
In a large glass or stainless steel bowl, whip the egg whites until they are foamy with stiff peaks.
Then while the egg whites are still whipping, gradually add the sugar and cornstarch mixture, 1 tbsp at a time, until the mixture becomes glossy and forms stiff peaks. The texture will be kind of like stiff shaving foam.
Line a baking sheet with a sheet of parchment paper.
Using a spoon, pile the meringue onto the parchment paper in a circle about 8 inches wide, and 3 inches tall. Make the sides of the meringue a little higher than the centre, leaving a slight bowl like indent in the middle...this creates a nice little spot for the lemon curd and whipped cream.
Bake for 1 hour and 30 minutes, until the pavlova meringue is barely golden in colour, has slight cracking and appears dry.
Set aside to cool. Let the meringue cool completely before adding the lemon curd and whipping cream.
While the Pavlova is baking, make the lemon curd.
In a saucepan whisk together the egg yolks, sugar, lemon juice and lemon zest.
Then on low-medium heat, cook the lemon mixture until it lightly boils then thickens. And make sure to stir it constantly. This step should only take about 5 - 8 minutes.
Remove the lemon mixture from the heat and whisk in the butter until it is melted and thoroughly mixed in.
Set aside to cool.
Using a mixer whip the whipping cream until it is fluffy. Note: I do not add sugar to this whipped cream because the pavlova and the lemon curd are sweet enough. The unsweetened whipped cream is a nice balance to all the sweetness.
Putting it all together:
Gently transfer the baked and cooled meringue base onto a serving plate.
Using a spoon gently add spoonfuls of the cooled lemon curd to the top of the meringue until it fills the centre. You can then use a spatula to smooth it out. Be careful not to press down hard or the meringue will collapse.