Whisk together the flour, zest , sugar and salt in the bowl of your food processor. Add the chunks of cold butter by disbursing them evenly over the flour mixture. Using short quick pulses, blend the butter with the flour until the texture is coarse and pebbly. Add the buttermilk and pulse until the dough comes together as a craggy mass. If necessary, add more buttermilk a tablespoon at a time until you achieve the desired texture.

Divide the dough in half. Wrap each half in plastic wrap and flatten it a bit, like a disc. Chill in fridge for at least an hour or up to two days.

You can also prepare your dough well in advance. Slip the plastic-wrapped dough into a freezer bag and freeze for up to 2 months. When ready to use, move the frozen dough to your fridge to defrost for one day.

Place rhubarb chunks and sugar in a small-to-medium saucepan. Stir to combine. Cover and cook at medium-low heat for 15 minutes, no need to stir. Remove the lid and Increase the heat to medium. Cook for another 10 to 15 minutes, until the rhubarb is broken down.

If you run a spoon across the bottom of the pot, you'll see a trench quickly form and disappear. That's the texture you're striving for. Transfer the mixture to a large plate and spread it so that it will cool quickly. Place the plate in the fridge or freezer to cool quickly, then scrape the mixture into a bowl. Keep cold until needed; it will be thicker and easier to “scoop” onto the pie bases.

Whisk the cream cheese, sugar, zest, juice and yolk together in a small bowl until smooth. Keep cold until needed as well.

Preheat oven to 400°F. Line two to three baking sheets with parchment paper. Beat the remaining egg and 1 tablespoon water and set aside with a pastry brush.

Flour the heck out of your work surface. Unwrap one of your dough rounds and start rolling your dough by pressing down lightly with the pin; moving it from the center out.

You’re not going to get it all flat in one roll or even twenty; be patient and it will crack less. Roll it a few times in one direction, lift it up and rotate it a quarter-turn. Continue to do, roll a couple times, lifting the dough and rotating it until the dough is about 1/8-inch thick.

Cut the dough into 2 3/4-inch to 3-inch squares. A crinkle-edged cookie-cutter works well here. A knife, pizza wheel or even a fluted pastry wheel would also do the job.

Carefully transfer the base squares to the prepared baking sheets, spacing them about one-inch apart. 

Repeat the rollout & cutout process with the second refrigerated dough disc. Cut a small crisscross vent in the second batch of squares. These will be your lids.

Try to work quickly while the dough is still cold from the fridge. If it warms up too much and becomes very soft and tacky, return the dough to the fridge for a few minutes of cooling. If you want to maximize the use of your dough, gather up all of the scraps and shape them into a flattened disc. Return that to the fridge to chill for a few minutes.

Brush the base squares very lightly with the egg wash. In the center of each egg washed square base, place a small dollop (a measured teaspoon) of cream cheese, then rhubarb filling on top. Be careful to not overfill them so that you have a clean edge all around for the seal.

Top each filled base with a vented square. Press the outer edge of top and bottom all around to seal with your fingertips; a fork will also do the trick. Brush the tops with egg wash and sprinkle generously with coarse sugar. 

Bake for 15 to 20 minutes, until puffed and golden, and even more brown at edges. Transfer to cooling racks and cool to room temperature before serving.