Whisk together the flour, powdered sugar, and salt.
2 c all-purpose flour, ¼ c powdered sugar, ⅛ tsp salt
Using a food processor (or by hand) add the butter to the flour mixture, and cut (using a pastry cutter or the blade of your processor) until it resembles coarse sand.
1 c cold butter
In a separate bowl, whisk together egg yolks and cold water until thoroughly combined. Stirring constantly, pour egg mixture into the dough, and mix until it begins to clump together.
2 large egg yolks, ¼ c ice-cold water
Lightly flour a portion of your counter and your (clean) hands. Take the dough and knead until it's nice and smooth (it only takes a few passes). Shape the dough loosely into a rectangle to make it easier to roll out later, then wrap it in an airtight wrap and chill in the fridge for at least an hour.
Meanwhile, set the cherries out to thaw, and drain the juices once they're no longer frozen.
For the Filling
Preheat oven to 350°F. In a small bowl, whisk together the sugar, tapioca, nutmeg, and salt. Combine all berries in a large bowl. Stir together in a small bowl. Add to berries; toss gently to coat.
1 ½ c granulated sugar, ¾ c uncooked quick-cooking tapioca, 1 tsp ground nutmeg, ½ tsp salt
In a large mixing bowl, stir together the thawed cherries and the sugar mix until the berries are evenly coated.
5 lbs frozen cherries
Pour the berry mix into a deep 9x13" baking dish, or a similarly sized aluminum pan. It's okay if the fruit is over the top of the pan by a half inch or so; it'll shrink while baking. Evenly sprinkle the cubed butter over the top of the berries and set aside while you roll out the crust.
3 Tbs unsalted butter
Roll out the cold dough onto a floured surface with a rolling pin until it's a few inches longer and wider than your pan. Use a small circle cutter, like a tiny cookie cutter or the base of a piping tip, to cut holes all over the dough, making sure to leave enough room for a crust at the edge.
Carefully pick up the dough and lay it on top of the cherry mixture. Cut off any excess around the pan with clean kitchen shears, then crimp the edges with a clean fork.
In a small bowl, whisk together the egg yolk and water. With a pastry brush, "paint" the egg yolk mixture all over the top crust.
1 large egg yolk, 1 tsp water
Bake for an hour and 15 minutes, or until the filling has slightly reduced and the crust is a crisp golden brown. If your crust seems to be cooking too fast, cover it with aluminum foil.
Once out of the oven, let the sonker cool at room temperature for about 2 hours to let the filling set.
For the Dip
In a medium sauce pan, stir together the half and half, sugar, and salt. Bring to a slight boil.
2 c half-and-half, ¾ c granulated sugar, ⅛ tsp salt
In a small bowl, whisk together the cornstarch and water until it forms a smooth paste. Stir into the boiling half and half.
1 ½ Tbs cornstarch, 1 ½ Tbs water
Stirring constantly, cook at a boil until the mixture is thick and bubbly, being careful to keep the milk from scorching.
Strain the mixture through a fine mesh strainer (in case there's a film on top) into a small pitcher or large jar.
Stir in the vanilla extract, and drizzle liberally over warm sonker.