In a saucepan melt butter and add brown sugar. Stir until the mixture bubbles up and looks slightly browner. Add milk to sugar stir to combine well. Whisk in egg yolks with flour and salt. Stir in remaining milk to mixture. When the sugar mixture becomes hot, slowly stir in the egg and flour mixture. Cook over medium heat until the pudding boils up. Turn the heat very low and stir and cook 1 or 2 minutes more. Remove from the heat and stir in the vanilla. Pour cooled butterscotch into pre-baked piecrust. Top the pie with meringue. Bake in a 350 degree preheated oven 15 to 18 minutes until lightly browned.
For the Crumb Crust: Put the metal chopping blade in place in the food processor beaker. Add the flour, frozen butter, and salt to the beaker. Process for 8 to 10 seconds, turning on and off, until the butter is cut into the flour and the mixture forms very small, flaky granules. Add the lemon juice and eggs to the beaker and process until a ball of dough forms on top of the blades, about 15 seconds. The dough should be damp but not sticky. If it seems too soft, sprinkle with 1 to 2 tablespoons flour and process for an additional 5 to 6 seconds, until well combined. If it feels too dry, sprinkle with a few drops of water and process to blend well. Wrap the ball of dough in waxed paper, plastic wrap, or foil and chill until firm but still malleable.
For the Meringue: Whip egg whites together with sugar until the peaks are soft. Rolling out the pastry. When the pastry is chilled and firm but still malleable, it is ready to be rolled out. Roll out on a cool surface as possible; a marble slab is ideal. Baking Shell -Blind Baking: Lay the roll out pastry dough into the pan, and line the shell with foil or waxed paper, weight it down with raw rice or beans and bake it in a preheated 425 degree oven for 14 to 16 minutes, until the bottom is set and the edges lightly browned. After the initial baking, remove the shell from the oven, remove the lining and brush the bottom with beaten egg yolk, and return to the oven for 2 minutes. This seals the bottom and prevents a soggy crust. Top the pie with the meringue and bake in a preheated 350 degree oven until the meringue is lightly brown.