Preheat oven to 350°. Place butter in a 2-quart baking dish; place in oven until butter melts, 3 to 4 minutes. Remove from oven.
In a medium bowl, stir together ⅔ cup granulated sugar, flour, bittersweet chocolate, 3 tablespoons cocoa, baking powder, and salt. Stir in milk and vanilla, stirring until smooth. Spoon over melted butter in dish (do not stir).
Stir together remaining ⅓ cup granulated sugar, brown sugar, and remaining 4 tablespoons cocoa. Sprinkle cocoa mixture evenly over batter. Pour 1½ cups hot water evenly over top (do not stir).
Bake until center is almost set but still jiggles and a wooden pick inserted in center comes out with some batter, 35 to 40 minutes. Let stand for 30 minutes.
Serve with whipped cream. Garnish with cherries, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/chocolate-cobbler/