Preheat oven to 350 degrees F. Roll out crust 1/8-inch thick. Cut into 5 (5-inch) rounds and fit into 3-inch tart pans with removable bottoms. Prick bottoms with a fork. Place a second tart pan inside of lined pan and place on a baking sheet. Bake 20 minutes. Remove top pan and continue baking until light golden brown, 5 to 10 minutes more. Remove from oven and allow to cool completely. Prepare butterscotch filling: Lightly beat egg yolks, set aside. Pass the brown sugar, flour, and salt through a medium sieve to remove any lumps. Transfer to a medium saucepan and place over medium heat to melt sugar, whisking frequently. Meanwhile, bring milk to a simmer. Whisk milk into melted sugar and cook, whisking constantly until thick, about 5 minutes. Remove from heat. Add 1/4 of the hot sugar mixture to the egg yolks and whisk to combine. Whisk egg yolks into sugar mixture and return to heat very briefly, just to cook the eggs and thicken slightly, about 30 seconds. Remove from heat and whisk in the butter. Pass mixture through a fine sieve. Prepare meringue: In the bowl of an electric mixer fitted with the whisk attachment beat egg whites until foamy, about 1 minute. Add granulated sugar and beat until stiff. Pour butterscotch filling into baked crusts. Mound meringue on top of tarts and bake until meringue is browned, about 10 minutes. Refrigerate until well chilled, at least 1 hour, before serving.
Sift the flour and salt together into a medium bowl. Add the shortening, and mix in with your fingertips just until the mixture resembles cornmeal. Add just enough ice water to form a dough. Gather into a ball, wrap in plastic, and place in refrigerator for a few minutes before rolling out. Makes 5 (3-inch) tart shells.
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