DOUGH: Use a stand mixer to cream the butter and sugar. Add the yolk and mix to combine. Add the flour and salt and blend.
Press the dough into a 9-inch tart pan (or springform pan) with a removable bottom and refrigerate for a minimum of one hour.
Preheat oven to 350 degrees.
Fill the tart shell with foil and pie weights. Bake 20-25 minutes or until the crust has a golden edge. Remove foil and continue to bake until the bottom of the tart shell is also golden.
CURD: Put the sugar, yolks, juice, zest, butter and salt in a large bowl.
Place over a pot of simmering water to achieve the effect of a double boiler. Stir with a spatula until the curd is thickened. Should take about about 10 minutes, possibly more. (The curd is ready when it does not run when you run your finger through it on the spatula.)
Use a fine mesh strainer to strain the curd, then cover with plastic wrap and refrigerate. Fill tart shell when cool.
MERINGUE: Stir together egg whites, brown sugar and salt in a CLEAN bowl from a stand mixer.
Stir over a double boiler until the sugar is melted and you cannot feel any sugar when you rub the mixture between your fingers.
Use mixer to whip until the mixture is room temperature and fluffy.
Use a large pastry bag or spatula to cover the lemon curd with the meringue.
To finish, use a kitchen-friendly blow torch (or broiler) to toast the meringue.