Combine the blackberries and sugar in a large saucepan and bring to a boil. Mix the cornstarch with 2 tablespoons water and add to the pan. Simmer until thick, about 5 minutes. Stir in the almond extract and orange zest, then remove from the heat and place in a 9-by-13-inch baking dish, allowing the mixture to cool slightly.
For the lattice crust: Preheat the oven to 425 degrees F.
Roll out the pie crusts on a lightly floured surface to approximately 10 by 14 inches. Cut one pie crust lengthwise into 4 long strips 1 3/4 inches wide (you will have scraps). Cut the second pie crust widthwise into 6 short strips, also 1 3/4 inches wide.
Place the 4 long strips vertically on a sheet pan with half an inch between them. Fold the first and third strips back on themselves and lay a short strip across. Fold the first and third long strips back to their original position over the top of the short strip. Now fold the long second and fourth strips back. Lay a short strip over and return the long strips. Repeat until you have laid six strips across and have a lattice. Trim the edges a little and put the sheet pan in the freezer for 10 minutes to allow the lattice to harden a little.
Place the lattice on top of the blackberry mixture. Make an egg wash by beating the egg together with a tablespoon of water in a small bowl. Brush the lattice with egg wash and bake until the crust is golden brown, about 30 minutes. (The berries may bubble over slightly, but that's normal.)
Serve bowls of cobbler with scoops of vanilla ice cream.