For the shells, combine the flour and salt. Cut in the shortening with a pastry blender until the mixture resembles coarse meal. Sprinkle cold water evenly over the surface. Stir with a fork until all the dry ingredients are moistened. Shape the dough into a ball and chill.
When chilled, roll the dough to 1/8 inch thickness on a lightly floured surface. Cut into 3 1/4 inch rounds. Fit each pastry round into 3 inch tart pans. Prick with a fork.
Bake in a preheated oven for 10-12 minutes. Let cool.
Place the butter in the top of a double boiler. Bring the water in the double boiler to a boil. Reduce the heat to low, and cook until the butter melts. Combine the sugar and cornstarch, stirring well. Add to the butter and stir until smooth. Stir in the eggs and cook for 5 minutes, stirring constantly.
Add the lemon juice. Cook 3-4 minutes longer or until the mixture thickens, stirring constantly. Cool. Spoon a rounded tablespoon of lemon filling into each cooled tart shell.
Before serving, top each tart with a dollop of whipped cream, if desired. Makes 22 tarts.