½cup (1 stick or 113 g)butter, cold and cut into cubes
5largeegg whites, at room temperature
1cup (200 g)granulated sugar
¼tspcream of tartar
Make and Bake Pie Shell
Make crust for single pie crust according to recipe instructions and blind bake it. Set aside to cool completely.
Combine the sugar, cornstarch, and salt in a medium saucepan and whisk to combine.
Add the lemon juice and water and bring to a near boil, whisking occasionally. Meanwhile, add the egg yolks to a large bowl and whisk to combine.
When the sugar syrup just begins to boil, remove it from the heat and slowly pour it into the egg yolks while whisking constantly to temper the egg yolks. Continue pouring and whisking until all the syrup has been added to the egg yolks.
Pour this mixture back into the medium saucepan and place over medium-low heat, whisking constantly, until mixture begins to boil. Boil for one minute, again whisking constantly. Remove from heat.
Add lemon zest to filling and whisk to combine. Add cold butter, one chunk at a time, until fully incorporated.
Pour filling into cooled pie shell. Cover with plastic wrap, completely touching the surface of the lemon filling. Let cool at room temperature for 30 minutes. Refrigerate for at least 6 hours, preferably overnight.
Make the Swiss Meringue
In the bowl of a stand mixer (or large heat proof bowl) set over a small saucepan filled with one inch of water, whisk egg whites, sugar, salt, and cream of tartar.
Place over medium heat, allowing water to simmer but not boil, and whisk constantly until mixture reaches at least 140° F (or up to 170° F). If you don't have a thermometer, you can test for doneness by rubbing a little bit of the mixture between your fingers. If there are no noticeable sugar crystals present and the mixture is completely smooth, it is ready. Mixture will be white and opaque.
Remove from the heat and begin whisking the mixture over high speed until doubled in volume and stiff peaks form (if you lift the whisk attachment from the bowl and turn it upside down, a "peak" will stand straight up).
Spread meringue over chilled pie, making organic swoops with a large spoon (if desired). Use a kitchen blow torch to toast the meringue all over. Serve immediately.
Pie will keep, uncovered, in the fridge for up to 3 days.