They key to a great cobbler is balance. Starting with the blackberry compote, you need to find the right ratio between lemon juice, cinnamon, and sugar, so that they complement the fruit without overpowering its delicate flavor. Then there's the dough, which should be flaky and light, like a cloud resting on top of the blackberries.
Luckily, cobblers require only a few easy steps and do not take very long to bake. Another great thing about this dessert is that it's extremy versatile — swap out the blackberries for whatever fruit is in season, and adjust the cinnamon for another complementary spice (like ginger if you're using pears).
Mix together the blackberries, sugar, lemon zest, lemon juice, cinnamon, and cornstarch in a bowl and let it sit for half an hour.
Preheat the oven the 350 degrees. In another bowl, mix together the flour, sugar, baking powder, and salt. Cut the butter into the mixture, using 2 knives or a dough blender — the dough should begin to form into crumbs. Stir in the egg and milk until everything is combined and the dough is uniform throughout.
Transfer the berries to an 8-inch round baking pan and spoon the dough on top. Bake the cobbler for 30 minutes, or until the fruit is bubbly and the dough starts to brown on top. Serve immediately.