1. Whisk together the flour, salt and baking powder in a large bowl.
2. Add the cubed butter to the bowl and use your fingers to work the butter into the flour until the mixture is the consistency of wet sand.
3. Stir in the sour cream (the dough will be very wet), and then turn the dough out onto a well-floured work surface.
4. Knead the dough a few times until it comes together, adding more flour, 1 teaspoon at a time, if the dough is too sticky to handle. Roll the dough into an 8x10-inch rectangle and dust both sides with flour before folding it into thirds as if you were folding a letter.
5. Rotate the dough 90o and roll it out again into an 8x10-inch rectangle. Fold the dough again into thirds then wrap it securely in plastic wrap and refrigerate it for 1 hour.
6. Preheat the oven to 425oF. Line two baking sheets with parchment paper.
7. Roll out the dough on a lightly floured work surface until it is about 1/8-inch thick. Cut the dough into squares about 2 1/2 inches in size. (The scraps can be re-rolled.) Place the pie crust squares onto the prepared baking sheets, spacing them 1 to 2 inches apart. Using a fork, prick the cookies all over then refrigerate them for 20 minutes. (See Kelly's Notes.)
8. Remove the baking sheets from the fridge. Whisk together the egg with 1 Tablespoon water then brush the tops of the cookies with the egg wash. Sprinkle the tops of the cookies with the sanding sugar (optional). Bake the cookies for 9 to 12 minutes until they're golden brown and cooked throughout. Transfer the cookies to a cooling rack to cool completely.
9. Once the cookies are completely cool, toast marshmallows over an open flame or under the broiler. Place a piece of chocolate on the bottom of one pie crust cookie, top it with the toasted marshmallow and place a second pie crust cookie on top. Serve immediately.
1. Refrigerating the pie crust cookies prior to baking helps guarantee they'll hold their square shape when baked.
2. The pie crust cookies can be baked up to 2 days in advance and stored in an airtight container at room temperature.