Begin the recipe by preheating the oven to 350 degrees F. and butter the bottom and sides of a 9-inch deep-dish pie plate.
In a medium bowl, whisk together the 2 cups all-purpose flour, 1 tablespoon baking powder, 3 tablespoons granulated sugar, ½ teaspoon salt, ¼ teaspoon ground cinnamon, and ¼ teaspoon ground nutmeg until combined.
Using a pastry blender, fork, or clean hands, add the 6 tablespoons cubed butter to the flour mixture and blend the butter into the flour mixture until the butter is about pea-sized and the flour mixture resembles a coarse meal.
Pour the ¾ cup heavy cream over the flour mixture and stir until it is fully combined and forms a soft dough. The dough will be soft and sticky, do not overwork or it will be tough. Turn the dough out onto a sheet of plastic wrap or wax paper and cover it with another sheet.
Use a rolling pin to gently roll the dough into a 9-inch round shape. Place the dough on a baking sheet, covered with plastic wrap, and place it in the refrigerator while preparing the blackberry filling.
In a large bowl, make the filling by gently stirring together the 6 cups fresh blackberries, ½ cup granulated sugar, 1 tablespoon cornstarch, and 1 tablespoon lemon zest until mixed together. Pour the filling into the prepared baking dish.
Remove the chilled dough from the refrigerator and cut a 2-inch wide hole in the center with a cookie cutter to create a steam vent. Carefully place the dough on top of the blackberry filling and brush lightly the top of the dough with heavy cream and evenly sprinkle with cinnamon sugar.
Place the pie into the oven to bake for about 55 to 60 minutes, until the top is golden and the fruit is bubbling. Carefully remove the cobbler from the oven and transfer it to a wire rack to cool for 30 minutes before serving. Serve the blackberry cobbler warm with a scoop of vanilla ice cream. Enjoy!