**flour amounts will depend on peaches; use more flour for juicy peaches.
Combine flour, salt and sugar in a large bowl. Add butter; cut in with pastry blender or two knives until mixture resembles coarse crumbs. Add milk, 1/4 cup at a time, stirring dough gently until mixture comes together.
Separate one third of the dough and form into a ball; form remaining 2/3 of the dough into another ball. Set aside and rest for 15 minutes. Roll out larger ball on lightly floured board into 14x17 rectangle. Ease dough over rolling pin and transfer to 9x13 inch baking pan, placing so dough covers the bottom and sides of the pan; set aside.
Preheat oven to 400. For filling, combine peaches, lemon juice, salt, cinnamon, allspice, brown sugar and granulated sugar into large bowl. Spoon fruit into pastry-lined pan; spread evenly. Scatter butter over fruit.
Roll out remaining dough into 9x13 inch rectangle. Place over fruit. Pinch edges of bottom crust over top crust, forming a ridge. Seal, cutting off and discarding excess dough. Cut 2 or 3 slashes into crust.
Place pan on baking sheet to catch any overflowing filling. Bake 1 hour or until crust is golden brown and fruit is bubbling. Remove from oven and cool at room temperature for 1 hour.
For glaze: combine powdered sugar and 1 1/2 tbsp milk and vanilla in small bowl. Add more milk if needed to reach consistency of pancake batter. Drizzle glaze onto peach squares. Set aside 30 minutes to set. Cut into 5 rows of 4 slices each.