Step 1In a medium bowl, combine the flour, sugar, salt and lemon peel, mixing together by hand. Cut the butter into the mix and work it, using your fingers or a pastry cutter, into the dry mixture until the whole resembles cornmeal. Add the water and vanilla extract and work until the dough holds together. Shape the dough into a flattened disc and wrap it in plastic wrap, leave it to chill in the fridge for 20 minutes or up to 5 days, or freeze for up to 6 months.
Step 1Heat the oven to 375 degrees. Form the tart shell. Press the dough into a tart pan and line it with a piece of parchment paper and weigh the pastry with pie weights. Bake the shell for 25 minutes, then remove from the oven. Carefully remove the parchment paper and pie weights, and set aside.
Step 2Grate the peel from the lemons, then juice the lemons and combine the peel and juice in a glass bowl.
Step 3Over medium heat, beat the eggs and the yolks with the sugar in a heavy, non-reactive saucepan just until mixed. Combine milk and cornstarch and add to the mixture, stir well and add the juice. Keep stirring as you add the butter, a little at a time and cook until the butter is combined and the mixture begins to thicken. Remove from heat. Set aside for 5 minutes to thicken and then whisk to smooth it.
Step 4Pour the filling into the prebaked tart shell and return to the oven for 20 minutes. Cool before serving.