Sprinkle with water, let rest 30 seconds and then either process very briefly or cut in with about 15 strokes of the pastry cutter, just until the dough begins to stick together and come away from the sides of the bowl. Turn onto a lightly floured work surface and press together to form a disk. Wrap in plastic and chill for at least 20 minutes.Allow the dough to warm slightly to room temperature if it is too hard to roll. On a lightly floured board (or countertop) roll the disk to a thickness of ⅛ inch (.3 cm). Cut a circle about 2 inches (5 cm) larger than the pie plate and transfer the pastry into the plate by folding it in half or by rolling it onto the rolling pin. Turn the pastry under, leaving an edge that hangs over the plate about ½ inch (1.2 cm). Flute decoratively. Chill for 30 minutes.

Preheat oven to 350ºF (180ºC). Line the crust with foil and fill with metal pie weights or dried beans.

Bake for 20 to 25 minutes. Carefully remove the foil and continue baking for 10 to 15 minutes, until golden. Cool completely before filling.

For the Filling:

Bring the water to a boil in a large, heavy saucepan. Remove from the heat and let rest 5 minutes. Whisk the sugar and cornstarch together.

Add the mixture gradually to the hot water, whisking until completely incorporated.

Add about 1 cup (240 mL) of the hot mixture to the beaten egg yolks, whisking until smooth. Whisking vigorously, add the warmed yolks to the pot and continue cooking, stirring constantly, until mixture comes to a boil.

Remove from the heat and stir in butter until incorporated. Add the lemon juice, zest and vanilla, stirring until combined. Pour into the prepared crust. Cover with plastic wrap to prevent a skin from forming on the surface, and cool to room temperature.

For the Meringue:

Preheat the oven to 375ºF (190ºC). Using an electric mixer beat the egg whites with the cream of tartar, salt and vanilla extract until soft peaks form. Add the sugar gradually, beating until it forms stiff, glossy peaks.