To make the crust:
Preheat the oven to 450ºF. In a food processor, combine the flour, salt, and sugar and process for 3 seconds. Add the butter and pulse until the mixture looks crumbly, with bits of dough the size of small grapes. In a small bowl, mix the sour cream and ice water together. Add the sour cream mixture to the flour mixture and give it 3 1-second pulses. On a floured work surface, form the dough into a ball and divide it in half. Gently mold the dough into 2 disks, cover with plastic wrap, and refrigerate for at least 2 hours. (The second crust can be refrigerated for up to 1 week or frozen for up to 1 month.)

Roll the dough out into a 1/8 inch thickness on a floured work surface. Fold it over the rolling pin and gently transfer the dough to a 10-inch deep-dish pie pan. Press it lightly into place, cut off any excess dough that falls over the edge of the pie pan, and crimp the edges with a fork. Line the crust with parchment paper or aluminum foil, fill it with dried beans r pie weights, and bake for 10 minutes or until the crust turns a light, golden brown. Remove from the oven, remove the paper or foil, and let the piecrust cool on a wire rack for at least 30 minutes before filling.

To make the custard:
In an 8-cup saucepan, whisk the milk and 1 1/2 cups of the sugar together over low heat. whisk in the egg yolks and flour and continue to heat while you make the caramel. Keep an eye on the milk mixture – do not let it boil. In a large heavy saucepan, cook the remaining 1 cup sugar over medium heat, stirring often until it melts and turns a deep golden brown, 20 to 25 minutes. Gradually pour the warmed milk mixture into the caramelized sugar and stir until smooth. (Watch for spattering.) Add butter, marshmallows, and vanilla. Stir until the butter and marshmallows are melted, then pour into the prepare piecrust. Let cool, then refrigerate the pie while making the meringue topping.

To make the topping:
Position an oven rack in the center of the oven. Preheat the broiler. In a large, clean stainless steel bowl, combine the egg whites and sugar. (Warning: if there is even a trace of fat in the bowl, the eggs won’t reach their proper volume.) Set the bowl over a saucepan with 2 inches of barely simmering water. whisking constantly, heat the mixture until the sugar melts and there is no visible grains in the meringue. Rub a little bit of the meringue between your fingers to make sure all the grains have melted. Remove from the heat and beat using an electric mixer on low speed for 5 minutes; increase the speed to high and beat 5 minutes longer or until the meringue is stiff and shiny.

Pile the meringue on top of the caramel custard. Style the meringue with your fingers or a spoon by plucking at it to tease the meringue into high, jagged spikes. Place the pie on the middle rack of the oven and boil until the meringue topping turns golden brown, about 1 minute. Watch the pie closely, as it can turn from browned to burned in a matter of seconds. If using a kitchen torch, hold it 2 to 3 inches away from the meringue and move the flame slowly around the meringue until it is allowed to brown all over. The pie should be served the day it is assembled.