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.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 regular-sized muffin pan.
On a lightly floured surface, roll out the dough to 1/4 inch thickness. Cut into 5-inch rounds. Line the prepared pans with the dough circles. Use a small kitchen knife to trim the excess and to score the tops of the tartlets.
Place in the refrigerator for about 15 minutes before baking for 10 to 15 minutes or until lightly golden.
Remove from oven and let the tart shells cool a few minutes before inverting and transferring them to a wire rack to cool completely.
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. Spoon the lemon curd into the lined muffin pan, and place it in the 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.
Transfer meringue to a piping bag attached to a large round tip. Pipe meringue on each tartlet. Use a kitchen torch to lightly brown the frosting.
Keep the tartlets in the refrigerator. Serve in the same day.