Separate the yolks from the whites and reserve the whites for the meringue. In a medium bowl add the yolks and whisk until lighter and smooth, a few minutes. Set aside.
Prepare an ice bath in a large bowl set aside. Place the bowl of a stand mixer in the freezer with the whisk attachment while you prepare the filling.
In a medium saucepan add the water, sugar, cornstarch, salt and lemon juice. Whisk to combine over medium high heat. It will be cloudy. The sugar should dissolve while whisking constantly. Bring to just a boil. Take a ladle full of the mixture and slowly add it to the egg yolks while whisking to temper the eggs. Pour the tempered eggs back into the saucepan while continuing to whisk until the mixture thickens and comes back up to a boil. Should take a few minutes more. Pour through a fine mesh strainer back into the medium bowl and gently place the bowl in the ice bath to stop the cooking. Whisk in the butter until melted and smooth. Whisk for a few minutes in the ice bath to continue to cool. Remove from ice bath and set aside.
Pull the bowl and whisk from the freezer and attach to the stand mixer. Pour the heavy cream into the chilled bowl and whisk on high until medium peaks form. This will only take a few minutes, don't over mix.
Take a third of the whipped cream and whisk into the cooled lemon curd to loosen. Switching to a spatula add the cream in three more additions, gently folding into the curd after each addition. Continue to gently fold until all the cream is fully incorporated into the curd.
Pour the lemon mousse into the pre-made crust and even off top. In a covered pie container or loosely covered not touching the mousse chill for at least 3 hours to overnight.
For the Swiss Meringue:
Fill a medium saucepan with about an inch or two of water to create a double boiler. Heat over high heat to bring to a bowl. Reduce heat medium low to gently simmer.
Using the reserved egg whites, plus one more egg white add to the clean bowl of a stand mixer. Add the sugar and salt and whisk. Place the bowl of the stand mixer over the saucepan but not touching the water. Whisk constantly until the sugar and egg white mixture is combined and lighter and when you rub a small amount of the mixture between your fingers there is no graininess. This should take about 8 to 10 minutes.
Return the bowl to the stand mixer with the whisk attachment and start off at medium speed for about 2 minutes to allow the steam to escape. Increase speed to high and whisk until stiff peaks form. Around 5 minutes or so. The bowl will be cooler to the touch.
Pull the pie out of the fridge and dump the meringue onto the mouse using an off-set spatula or knife to spread out across the pie. Can be as uneven as you want and higher in the center.
Carefully toast the top of the meringue until browned (if you don't have torch, carefully toast in the oven under the broiler on low for a few minutes. Watch and don't walk away). Slice and serve.
Can store leftovers in airtight container in the fridge for up to 2 days.
Enjoy!
Recipe by Social Buttercream at http://socialbuttercream.com/2020/09/19/lemon-meringue-mousse-pie-pucker-up/