To prepare crust, sift the flour and salt in a large bowl. Set aside.
Cream butter and powdered sugar in a bowl of a standing mixer until light and fluffy. Beat in vanilla.
Reduce the speed to low and beat in the flour mixture until just incorporated.
Gather dough and form a dish. Cover with plastic wrap and refrigerate for about 30 minutes to chill.
Preheat oven to 350F. Grease a tart pan with a removable bottom.
On a lightly floured surface, roll out the dough to 1/4 inch thickness. Line the prepared pan with the rolled dough. Leave a tiny overhand of dough. Use a small kitchen knife to trim the excess and to score the tops of the tart.
Bake the crust for 15 to 20 minutes, or until the edge is lightly browned.
Remove from oven and let the crust cool completely before removing from the pan.
To prepare lemon curd, mix lemon zest and sugar with a fork.
In a large bowl, cream the butter. Add the sugar mixture and beat together. Add the eggs, lemon juice and salt and mix together until thoroughly combined.
Pour the mixture into a pan and cook over low heat while stirring constantly for about 10 to 15 minutes or until the curd starts to thicken and coat the back of a wooden spoon.
Remove from heat. Push the lemon curd through the sieve and into a bowl to make it super smooth. Pour the lemon curd into the cooled tart shell, and place it in refrigerator to chill for at least an hour.
To prepare meringue, in a large heatproof bowl, combine sugar and egg whites. Using a handheld electric mixer, beat on high speed until foamy for 1 minute. Set bowl over a pan of barely simmering water. Beat on high speed until the mixture is hot to touch and sugar is dissolved. Remove from heat, stir in water, and continue whipping for another 10 minutes until stiff and glossy peaks form.
Place meringue on top of the filling. Use a kitchen torch to lightly brown the frosting.
Keep the tart in the refrigerator. Serve in the same day.