How to Make It
Weigh or lightly spoon 9 ounces flour into dry measuring cups, and level with a knife. Combine flour, 1 tablespoon sugar, and 1/2 teaspoon salt in a bowl; cut in 6 tablespoons butter with a pastry blender or 2 knives until the mixture resembles coarse meal. Sprinkle surface with juice and ice water, 1 tablespoon at a time; toss with a fork until moist and crumbly (do not form a ball). Gently press two-thirds of dough into a 4-inch circle on plastic wrap, and cover. Press remaining dough into a 4-inch circle on plastic wrap, and cover. Chill 3 hours or overnight.
Preheat the oven to 425°. Place a foil-lined baking sheet on middle rack.
To prepare filling, combine 1 1/4 cups sugar and next 4 ingredients (through allspice) in a small bowl, stirring with a whisk. Combine plums and peaches in a large bowl; sprinkle flour mixture over plum mixture. Drizzle with vanilla and melted butter; toss gently.
Working with large portion, roll dough out on a lightly floured surface into a 13 x 9-inch rectangle. Fit into a 2-quart glass or ceramic baking dish coated with cooking spray. Spoon plum mixture into crust. Roll the remaining portion of dough into an 11 x 7-inch rectangle. Cut dough lengthwise into 1/2-inch-thick strips; arrange strips in a lattice design over plum mixture. Seal dough strips around edges of crust. Lightly brush strips with milk; sprinkle evenly with remaining 1 tablespoon sugar. Place baking dish on preheated pan; bake at 425° for 20 minutes.
Reduce oven temperature to 375° (do not remove cobbler from oven). Bake for 50 minutes or until crust is golden and filling is bubbly. Cool at least 1 hour.