Combine the flours, sugar and salt in the bowl of a food processor. Pulse to combine. Add the cold butter cubes to the bowl and pulse until the butter is incorporated into the flours and largest bits look to be the size of peas.
Then, with the motor running, slowly stream the water into the bowl using the tube. Stop once you’ve added 1/4 cup of water and test the dough by squeezing it. If it sticks together, it’s done. You want it to just barely hold together.
Divide the dough in two and wrap it in plastic wrap or waxed paper. Store in the refrigerator for at least an hour before using. Overnight is fine too. The dough can also be frozen for up to a month.
If you don’t have a food processor, pie dough is still within your grasp. Combine the flours, sugar and salt in a large bowl and whisk together. Grate very cold butter using a box grater. When it’s all grated, combine with the flours in the bowl and work together using a pastry blender or your hands. Add water drop by drop until the dough comes together. Divide and store as recommended above.
Pit sour cherries and let sit for half an hour (I find that freezing and then defrosting the cherries is a really effective way to remove some of the excess liquid). Drain any liquid off. Add sugar and flour to cherries and stir. Set aside.
Roll out the pie crust and cut six rounds to fit your muffin tin or custard cups. Gently fit the rounds into the muffin wells. Evenly divide the cherry pie filling between the six cups. Top each small pie with 1/2 tablespoon butter.
Cut six smaller rounds of pie crust to use as lids. Cut vents to allow for steam (or use your creativity to devise something more aesthetically appealing). Brush pastry lids with egg wash on both sides. Fit onto pie bottoms and crimp to seal.
Bake at 375 degrees for 35-40 minutes, until tops are browned and the pie juices run thick and glossy.
Remove from oven and allow to cool for at least 10 minutes before removing from tins. Eat immediately or freeze on a cookie sheet.