Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or non-stick spray.
In a large mixing bowl, whisk together the granulated sugar, flour, cocoa powder, baking powder, and salt until well combined.
Stir in the milk, melted butter, and vanilla extract until the batter is smooth and fully incorporated. Pour the batter into the prepared baking dish.
In a separate bowl, mix the brown sugar and hot water until the sugar is dissolved. Carefully pour this mixture over the batter in the baking dish. Do not stir; it will create a sauce underneath the cobbler.
If using, sprinkle the chopped pecans evenly over the top.
Bake in the preheated oven for 30-35 minutes, or until the top is set and a toothpick inserted comes out with moist crumbs.
Allow the cobbler to cool for about 10 minutes before serving. It is best enjoyed warm with a scoop of vanilla ice cream on top.