Add the flour and salt in a large bowl and whisk to combine. Add the cubed butter and use your fingers to smush the butter into flat pieces. Continue smashing the butter until all of it is flattened and pieces are about the size of quarters. The flour should look like coarse cornmeal.
Add about 4 tablespoons of water initially and quickly fold it in with a wooden spoon to distribute the water. The dough should appear shaggy. If you need to add more water, add it one (1) tablespoon at a time until it appears shaggy (you may not need all of the water). Keep the dough in the bowl and use your fingers or the palms of your hand to smear the dough up the sides of the bowl.
Once you've done this all the way around the bowl. fold the dough over itself to pick up any dry bits from the bottom of the bowl. Pour the dough onto a floured work surface (I use a wooden board). Press the dough into a rectangle, lightly flour the top and use a rolling pin to roll out to a rectangle, approximately, 10 x 8-inches. Use a bench scraper to fold the dough into a letter fold. Repeat the rectangle and the letter fold two more times.
Divide the dough into two pieces and wrap each piece in plastic wrap. Chill the dough for at least 30 minutes. The dough can be refrigerated for up to 1 week or frozen for up to 1 month.
Rhubarb Filling
Chop the rhubarb into 1/2-inch pieces. Place the sugar and water in a 3-quart saucepan and bring it to a boil, only stirring enough to dissolve the sugar. Reduce the heat to medium and cook until the caramel is a light amber color. Remove the pan from the heat and add the chopped rhubarb. Some of the caramel will harden, but that is okay. As the liquid from the rhubarb is released the hard pieces of caramel will melt.
Return the pan to the heat and stir continuously over medium-low heat until the rhubarb is broken down and soft. The final mixture will resemble a puree. If the rhubarb starts to stick to the bottom of the pan as it is cooking, add more water 1 tablespoon at a time. When the rhubarb is done taste it and if it is too tart for your tastes add more sugar 2 tablespoons at a time until the desired sweetness is reached.
Scrap the rhubarb into a bowl and allow it to cool to room temperature before making the hand pies. If not making the hand pies the same day, refrigerate the rhubarb until read to use.
Mascarpone Filling
Place the mascarpone, kosher salt, and sugar in a small bowl and zest the lemon directly into the bowl. Use a spatula to mix the ingredients together. Mix in the egg yolk. Add the lemon juice and sour cream and stir until combined. Refrigerate until ready to assemble the hand pies.
Assemble the Hand Pies
Take one half of the dough and let it sit at room temperature for 10-15 minutes. This step will make it easier to roll out. Roll the dough out to 1/8-inch thickness.
Use a 3-inch square cutter to cut out the squares. Place 1 teaspoon of the mascarpone filing in the center of the square and 1-1/2 teaspoons of the rhubarb on top of the mascarpone. Brush the edges of the square with egg wash and place a second dough square on top, pressing the two halves together. Use the tines of a fork to seal the edges together.
Gather of the dough scraps and press into a dough circle. Wrap in plastic wrap and refrigerate. You will be able to get another 5-6 pies from the dough scraps.
Repeat the assembly steps on the second half of dough. When the pies are assembled refrigerate them for at least 30 minutes and up to 2 days. The hand pies can also be frozen at this point. Place them on sheet tray in a single layer, freeze them and then place them in a freezer bag.
Baking the Rhubarb Hand Pies
30 minutes before you’re ready to bake the pies, preheat the oven to 400°F. Place 6 pies on a half baking sheet lined with parchment paper. Brush the pies with egg wash, cut an opening into the top and then sprinkle them with turbinado sugar. Bake them for 18-20 minutes until dark golden brown. If you want them darker bake for another 2 minutes.
Remove the sheet from the oven, cool the pies for 5 minutes and then move them to a cooling rack. The pies taste absolutely amazing while they are still warm.
Storage
The pies are best on the day they’re baked. Store the pies at room temperature in an air tight container for 2 days and in the refrigerator for up to a week. Rewarm the pies in a 350°F oven.
Notes
When baking the hand pies, don't overcrowd the pan. The space between the pies allows for good heat circulation and good browning on the sides and bottoms.