Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.
In a mixing bowl, combine the blackberries and sliced peaches. Add ½ cup of the sugar, cornstarch, vanilla extract, and lemon juice. Toss well to coat the fruit evenly. Pour the fruit mixture into the prepared baking dish.
In another large bowl, whisk together the flour, baking powder, salt, remaining ½ cup of sugar, and ground cinnamon.
Pour the melted butter and milk into the dry ingredients. Mix just until combined; the batter will be lumpy.
Spoon the batter evenly over the fruit mixture, making sure it’s distributed as evenly as possible. Do not mix it into the fruit.
Bake in the preheated oven for about 45-50 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Remove from the oven and let cool for a few minutes before serving.
Notes
Serve warm with a scoop of vanilla ice cream on top, and drizzle a little extra syrup from the cobbler around the plate for a beautiful presentation.