How to Make It
In a food processor, combine the flour, sugar, lemon zest and salt and pulse to blend. Add the butter, egg yolks, vanilla, orange juice and rum and pulse until the dough begins to clump together. Transfer the dough to a work surface, gather it up and roll it into a thick cylinder. Cover well with plastic wrap and refrigerate for at least 4 hours.
In a small nonreactive saucepan, combine the lemon juice, sugar, lemon zest and 1/3 cup of water. Bring to a boil over moderate heat, stirring occasionally to dissolve the sugar. Whisk in the cornstarch mixture and continue whisking the mixture until bubbling and thickened, about 1 minute. Remove from the heat and whisk in the butter and egg yolks. Strain the filling though a fine stainless steel sieve into a glass or ceramic bowl. Let cool to room temperature, then cover and refrigerate.
Lightly spray six 4- or 4 1/2-inch tart shells with vegetable oil cooking spray. Cut the cylinder of pastry into 6 equal pieces. On a lightly floured work surface, roll one piece of the pastry into a 6-inch round and carefully fit it into one of the tart shells, smoothing out any cracks. Repeat with the remaining pastry. Trim the overhanging pastry flush with the rims of the tart pans.
Preheat the oven to 400° and set a heavy baking sheet on the middle rack. Prick the bottoms of the tart shells all over with a fork and line each one with foil. Fill with dried beans, rice or pie weights and bake the shells on the baking sheet for 15 minutes, or until the edges are golden. Remove the foil and dried beans and bake for about 8 minutes longer, or until the shells are golden brown and beginning to pull away from the sides of the pans. Transfer the shells to a rack and let them cool slightly.
Turn the oven down to 350°. Spoon the lemon filling into the tart shells and bake for 5 minutes. Let cool at room temperature for a few hours until the filling firms up, or refrigerate the tarts for up to 4 hours before serving.