Preheat the oven to 350 degrees.
While the oven is preheating, add butter to a 9x13 casserole dish, and place it in the oven to melt-about 3-4 minutes. Once melted, remove the dish and set it aside.
Stir together granulated sugar, flour, and 2 tablespoons cocoa powder. Add milk and vanilla and whisk until smooth.
Pour batter mixture over butter. Do Not mix the butter with the batter.
Combine brown sugar and remaining cocoa powder and pecans. Sprinkle over the batter.
Slowly pour hot water carefully over the top. Do Not stir.
Bake for 35-40 minutes or until the cobbler is set on top.
Serve warm with vanilla ice cream and chopped pecans, if desired.