To make the pie dough: In a large bowl, whisk together the flour, sugar, and salt. Add the butter cubes and gently toss to coat them in flour. Squeeze a couple cubes of butter at a time until each cube has been flattened. Continue squeezing and flattening small handfuls of the butter and flour until the butter breaks down into pea-sized pieces.  , In a measuring cup, combine 1/4 cup (56g) water and the vinegar. Slowly pour this liquid into the bowl of the flour mixture while gently stirring with a fork. Stir until the bottom of the bowl is mostly clear of dry flour. If necessary, add additional water by the tablespoon; the dough should hold together when squeezed, without excessive dry bits at the bottom of the bowl. , Carefully transfer the dough to a clean surface and pat it into a rough 1" square. With a bench knife, cut the square in half. Place one piece on top of the other, then pat the dough back down into a 1" square. Repeat this process of cutting, stacking, and patting 2 more times; this creates flaky layers. If dry bits of dough fall away from the edges of the square, use the bench knife to scoop and drop them between the dough layers before you stack and press. By the end, the dough should be more uniform, with slightly ragged edges and small streaks of visible butter.  , Lift up the block of dough and lightly sprinkle flour underneath. With a lightly floured rolling pin, gently roll the dough into a 1" thick circle, then wrap tightly in plastic or in your favorite reusable wrap and store in the refrigerator for at least 3 hours or in the freezer for at least 1 hour to firm up the butter.   , When ready to bake, preheat the oven to 425°F with a rack in the middle position. In a spice grinder or food processor, combine the granulated sugar and black tea. Grind the mixture until the tea is practically indistinguishable from the sugar. Reserve 1 tablespoon in a small bowl. , Add the remaining tea and sugar mixture to a large bowl with the cornstarch and salt; whisk to combine. Add the peaches, lemon juice, and vanilla and mix until the peaches are evenly coated. Transfer the mixture to a 10" cast iron skillet.  , Unwrap the chilled dough and place it on a lightly floured surface. (If necessary, let it sit for 5 to 10 minutes to temper.) Dust the top of the dough with more flour and roll it out to a 12" to 13" circle between 1/8" and 1/4" thick. Drape the dough over the skillet, pressing gently so that it touches the peaches. Use kitchen scissors to trim the dough so that there’s no more than 1" of overhang, then tuck this into the pan. Trim the dough so that it’s flush with the top of the skillet; avoid any excess dough hanging over the edges or rising higher than the sides of the skillet. , Brush the dough all over with egg wash, then cut or snip 4 to 5 large vents in the center of the dough. Mix the coarse sugar into the reserved tea and sugar mixture, then sprinkle evenly over the dough. Place the skillet on a rimmed baking sheet.  , Bake the pandowdy for 35 minutes, until the crust is starting to brown in spots but still looks mostly pale and underbaked. Remove the skillet from the oven and use a sharp knife to cut through the crust to create large irregular pieces of dough. Use the back of a spoon to gently nudge some pieces downward to meet the filling. , Return the skillet to the oven for 10 to 15 minutes until the crust is deeply brown and the filling is bubbling. Allow the pandowdy to cool for 30 minutes before serving.  , Storage information: Pandowdy is best served warm, soon after it’s baked. Leftover pandowdy can be stored in a sealed container in the refrigerator for 3 days. Reheat in the microwave in 30 second bursts or in a 300°F oven for 15 to 20 minutes.