In a small bowl, combine flour, cornstarch, and salt. Set aside.
In the bowl of a stand mixer, combine the butter and sugar. Beat until light and fluffy. Scrape down the sides of the bowl.
Add egg and mix to combine. Scrape down the sides of the bowl again.
Add flour mixture and mix until barely combined. Pour the mixture out onto the countertop and gently knead into a ball. Form the dough into a disc, wrap in plastic, and refrigerate for one hour.
Grease a 9" tart pan with removable bottom (see note) and set aside.
After chilling, turn the dough out onto a lightly floured surface. Roll to about ¼" thickness and line prepared tart pan. You will have leftover dough (great for making garnish or saving for mini tartlets.) Place the crust in the refrigerator to chill while the oven preheats to 350° F.
Place the chilled crust on a baking sheet. Line with foil and fill with pie weights (see note). Bake 10 minutes, then remove foil/weights and bake for an additional 10 minutes. Edges of the crust should be golden. Allow crust to cool.
For Filling:
In a large bowl, combine all filling ingredients and whisk to combine. Pour filling into the baked crust.
Bake 20-30 minutes, or until the filling is nearly set (it will still wiggle a little in the center when you shake the pan). If overcooked, the filling will crack. Allow to cool completely on a wire rack.
For Meringue:
Pour egg whites into the bowl of a stand mixer fitted with the whisk attachment. Move on to making the sugar syrup.
In a small saucepan, combine sugar and water. Bring mixture to a boil over medium-high heat. Continue to cook until mixture reaches 240° F (see note). At this point, turn the mixer on medium-high speed to start whipping the egg whites. Continue cooking the sugar syrup until it reaches 245° F. (The idea is to have the egg whites whipped to soft peaks at the same time the sugar syrup comes to temperature.)
With the mixer on low speed, *carefully* drizzle the sugar syrup into the whipped egg whites. Once all the syrup is added, increase the mixer speed to high and whip until cooled. Meringue will be white and fluffy.
Transfer meringue to a piping bag to pipe on top of the cooled tart, or dollop the meringue on with a spoon. Toast meringue with a torch or under the broiler (see note).Enjoy!
Notes
Note on tart pan: I used a 9" tart pan with removable bottom, like this one [here].Note on pie weights: Pie weights really do help when blind-baking a crust, helping to keep the dough from rising/getting bubbles as it bakes. This makes sure that you have the room that you need for filling. You can buy actual pie weights if you'd like (like these here), but I just use black beans. They're cheap and you can use them over and over again. :)Note on candy thermometer: You will need a candy thermometer to ensure your success with the meringue! If you don't have one, you can find one [here].
Note on toasting meringue: I use a kitchen torch to toast my meringue. If you don't have a torch, you can put the tart under the broiler, just don't take your eyes off it for a second! I prefer the torch, since it gives more control over the toasting. My torch is old so I don't even know what kind it is, but this one (link [here]) has great reviews.