Triple Berry Pies with Buttermilk Crust

Steps

1. Make the crust: whisk the flour, 7 g of the salt and 25 g of the sugar in a large bowl. Toss in the cold butter and press the cubes into flat pieces with your fingertips, leaving some the size of peas.
2. Stir the ice-cold buttermilk and ice-cold water together, drizzle over the flour mixture and toss with a fork until the dough holds together when squeezed.
3. Divide the dough into 4 equal pieces, flatten each into a disk, wrap and chill for at least 1 hour.
4. Make the filling: in a large bowl, whisk 200 g of the sugar with the cornstarch, the cinnamon and the remaining 2 g of salt. Add the blueberries, raspberries, strawberries, lemon juice and vanilla and fold gently. Let stand 15 minutes while you roll the dough.
5. Set an oven rack in the lower third with a rimmed baking sheet on it and preheat to 425°F (220°C).
6. Roll 2 dough disks into 12-inch rounds and fit each into a 9-inch pie plate, leaving the overhang. Divide the berry filling and its juices evenly between the two crusts.
7. Roll the remaining 2 disks into 11-inch rounds and lay one over each pie. Trim, fold the edges under, crimp, and cut 5 or 6 steam vents in each lid.
8. Brush the lids with the beaten egg and sprinkle with the remaining 20 g of sugar. Chill the pies 15 minutes.
9. Bake the pies on the hot baking sheet for 20 minutes, then lower the oven to 375°F (190°C) and bake 45 to 55 minutes more, until the crust is deep golden and the filling bubbles thickly through the vents; tent the edges with foil if they brown too fast.
10. Cool the pies on a rack for at least 4 hours so the filling sets, then cut each pie into 6 slices.