In food processor, pulse together flour, sugar, and salt.
Sprinkle in chopped, cold butter and pulse until coarse crumbs begin to form.
Add sour cream, pulse until mixture begins to clump.
Pour dough out onto clean, well-floured surface and lightly dust with more flour (mixture will likely be very sticky).
Knead dough, adding flour as needed until it is cohesive and no longer sticky (you may need to add as much as ¼ cup flour).
Using your palm, flatten the dough on your clean surface.
Fold dough into itself, repeat flattening process (this helps encourage flaky layers in the crust) and then form dough into a disk, wrap tightly with clear wrap and place in fridge to chill for at least 30 minutes.
After 30 minutes, remove dough from refrigerator and preheat oven to 400F.
Remove clear wrap from dough and turn dough out onto lightly floured surface.
Use a rolling pin to roll dough to approximately 18"x18" square (approximately ⅛" thickness, if you do not have enough space for an 18"x18" square, divide the dough in half and roll out to ⅛" thickness, one at a time.)
Using a biscuit cutter or a glass with a 3" diameter rim, cut out circle-shaped pieces of dough and place half of them on parchment paper-lined cookie sheets and set the other half aside (these will be your top layers).
Once you have cut out all you can from the dough, gather the scraps and knead them together and then roll out into a ⅛" thick square again to cut from that, repeating until you have no scraps left.
Filling
In small saucepan, heat chocolate chips and condensed milk on medium-low heat, stirring until chocolate chips are melted.
Reduce heat to low (as low as possible) and stir in vanilla extract.
Portion 1 tsp of marshmallow fluff on top of the pastry circles on the cookie sheet.
Gently portion 1-1 ½ tsp milk chocolate mixture over the marshmallow, taking care to keep the filling in the center.
Place the set aside pastry circles over the filling and press down around the edges with the tines of a fork to seal in the filling.
Gently beat together the egg and the water for an egg wash and brush over prepared hand pies.
Use a sharp, non-serrated knife to cut vents in the top centers of the hand pies and then sprinkle with graham cracker crumbs.
Bake on 400F for 15 minutes, allow to cool at least 10 minutes before eating.
Notes
If too much filling is added, it may burst out of the seams of the hand pies. If allowed to cool, the top layer of the hand pie will usually settle back down over the exposed filling, though. These hand pies are best served warm and fairly fresh, as with regular s'mores.