Make the Rhubarb Filling:

1. Melt butter in a medium saucepan. Add the rhubarb and cook on medium-low heat until soft and broken down. Stir in sugar, vinegar, lemon juice, and salt. Cook for an additional 3 minutes. Let cool to room temperature before using.

Make the All-Butter Pie Crust:

1. Stir the flour, salt, and sugar together in a large bowl.
2. Add the butter pieces and coat with the flour mixture using a bench scraper or spatula. With a pastry blender, cut the butter into the flour mixture, working quickly until mostly pea-sized pieces of butter remain (a few larger pieces are OK; be careful not to over blend).
3. Combine the water, cider vinegar, and ice in a large measuring cup or small bowl.
4. Sprinkle 1/4 cup of the ice-water mixture over the flour mixture, and mix and cut it in with a bench scraper or spatula until it is fully incorporated. Add more of the ice-water mixture, 1 to 2 tablespoons at a time, using the bench scraper or your hands (or both) to mix until the dough comes together in a ball, with some dry bits remaining.
5. Squeeze and pinch with your fingertips to bring all the dough together, sprinkling dry bits with more small drops of the ice-water mixture, if necessary, to combine.
6. Shape the dough into a flat disc, wrap in plastic, and refrigerate for at least 1 hour, preferably overnight.
7. Wrapped tightly, the dough can be refrigerated for 3 days or frozen for up to one month.

Assemble the Hand Pies:

1. Preheat oven to 350° Fahrenheit.
2. Roll chilled pie dough out in a rectangle shape on a floured surface to approximately 1/8 inch thick.
3. Cut dough into forty 4-inch squares, re-rolling if necessary.
4. Top with a spoonful of the rhubarb filling.
5. Cover with another pie dough round and gently pinch the edges together.
6. Use the tines of a fork to seal further and to create a fluted edge. Cut a small X-shaped slit on the top of each pie with a paring knife.
7. Use a pastry brush to brush with heavy cream and sprinkle with raw or demerara sugar.
8. Chill in freezer for 10 minutes before baking.
9. Bake for 25 to 35 minutes until dark golden.