Prepare Butterscotch: In a medium bowl, lightly beat egg yolks; set aside. Pass 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, in a small saucepan, bring milk to a simmer. Whisk milk into melted sugar, and cook, whisking constantly until thick, about 5 minutes. Remove from heat. Add one-quarter of the hot sugar mixture to beaten egg yolks, and whisk to combine. Whisk egg yolks into sugar mixture, and return to heat very briefly to cook the eggs and thicken slightly, about 30 seconds. Remove from heat, and whisk in 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 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.

                Pie Crust
                Makes five 3-inch tart shells

                1 1/2 cups all-purpose flour
                1 teaspoon salt
                1/4 cup solid vegetable shortening
                4 to 6 tablespoons ice water

Sift flour and salt together into a medium bowl. Add shortening, and mix in with fingertips 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.