Whisk together flour, sugar, cocoa powder, baking powder, and salt. Stir in milk, melted butter, and vanilla extract just until combined. Stir in chocolate chips and walnuts, if using. Spread into the bottom of an 8-inch baking dish.
1 cup (141g) all-purpose flour, 2 teaspoons baking powder, ½ teaspoon salt, ¾ cup granulated sugar, 2 Tablespoons cocoa powder, 4 Tablespoons salted butter, ½ cup milk, 1 teaspoon pure vanilla extract, ½ cup chopped walnuts, ½ cup semisweet chocolate chips
Combine brown sugar, granulated sugar, cocoa powder, and salt in a small bowl. Sprinkle over the top of the cobbler batter. Pour boiling water over the batter and DO NOT STIR. Immediately place the cobbler into the oven.
¼ cup granulated sugar, ½ cup brown sugar, ¼ teaspoon salt, ¼ cup cocoa powder, 1 cup boiling water