Preheat the oven to 350 degrees. Roll out dough to 1/8-inch thickness. Cut into 5 five-inch rounds, and fit into 3-inch tart pans with removable bottoms. Prick bottoms with a fork. Place a second tart pan inside each dough-lined pan, and place them all on a baking sheet; bake 20 minutes. Remove top pans, and continue baking until light-golden brown, 5 to 10 minutes more. Remove from oven, and allow to cool completely.
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 1/4 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 shells. 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.