Prepare the tart shell. Cut the butter into ½-inch cubes and refrigerate. Meanwhile, mix together the flour and salt. In a separate cup, mix the egg yolk and heavy cream. In a food processor, place cold butter cubes and sugar and pulse until sugar coats the butter. Add the flour mixture and pulse until the mixture resembles breadcrumbs. Add egg mixture and pulse until incorporated. The dough will look crumbly. Scrape the dough out onto a sheet of plastic wrap. Using the plastic wrap to knead the dough, form into a disc. Wrap it with more plastic wrap and refrigerate for 30 minutes.
Preheat the oven to 325°F.
Roll the dough about 1/8-inch thick and transfer it to the tart pan; gently press into the bottom and up the sides of the tart pan. Line inside of pastry with heavy duty foil.
Bake for 10 minutes; remove from oven. Remove foil, prick bottom and sides of pastry, and return to oven for an additional 4 minutes or until golden brown. Cool slightly before filling.
In medium saucepan, combine sugar, cornstarch and salt. Gradually stir in water and cook, stirring constantly, 2 to 3 minutes, until mixture is clear and thickened. Remove from heat.
In a separate bowl, combine egg yolks, lemon zest and, juice; stir into hot sugar mixture. Cook, stirring constantly, 3 minutes longer. Remove from heat and stir in butter; cool. Pour into prepared tart shell.
In the bowl of an electric mixer fitted with the whisk attachment, place the egg whites, cream of tartar, and salt and beat on medium speed until foamy. Gradually sprinkle in the sugar, beat at medium/high speed until incorporated, and stiff glossy peaks form.
Spoon the meringue over the tart. Use the back of a spoon or a spatula to make peaks with the meringue. Using a kitchen gas torch, brown the meringue. If you do not have a torch to brown the meringue, turn on the oven to broil and brown the meringue for 30 seconds or until golden brown. Meringue will brown quickly so stay close to the oven until it reaches the desired browning.
Refrigerate for at least 3 hours. Serve.