In a large mixing bowl, combine the flour, granulated sugar, and salt.
Add the cold, cubed butter to the dry ingredients. Using a pastry cutter or your fingers, mix until the mixture resembles coarse crumbs.
Stir in the cold water, adding one tablespoon at a time, until the dough comes together. Be careful not to overwork it.
Divide the dough into two equal disks, wrap in plastic wrap, and refrigerate for at least 30 minutes.
Preheat your oven to 375°F (190°C) and line two baking sheets with parchment paper.
Once chilled, roll out one disk of dough on a lightly floured surface to about 1/8-inch thick. Cut out 3-inch circles using a cookie cutter or a glass.
On one half of each circle, place a few mini marshmallows, a sprinkle of crushed graham crackers, and a few chocolate chips.
Fold the other half of the dough over the filling to create a semi-circle. Press the edges together to seal, then crimp with a fork for an added decorative touch.
Place the hand pies on the prepared baking sheets and brush the tops with the beaten egg.
Bake in the preheated oven for 20-25 minutes or until the hand pies are golden brown.
Once baked, let them cool slightly before serving.
Notes
Serve warm on a rustic wooden board, dusted lightly with powdered sugar and accompanied by a side of melted chocolate for dipping.