Preheat oven to 350ºF.: As the oven warms, you will notice a dry, warming scent and the elements preparing to transform the batter into a golden top, which is essential because the steady initial heat promotes even rise. If you skip preheating, the batter can bake unevenly and produce a dense center, so always wait until the oven reaches a stable 350 degrees.
In a medium bowl whisk together 3/4 cup sugar, flour, baking powder, salt, milk, 2 Tbsp cocoa powder and vanilla.: Right away you will feel the dry ingredients combine into a light, sandy texture before the wet ingredients bind them into a smooth batter; the aroma of cocoa and vanilla becomes noticeable and reassuring. The whisking develops small air pockets that encourage a tender crumb, so take time to mix until smooth but avoid over whisking which can toughen the cake. A frequent error is adding wet to dry too quickly, which can create lumps, so pour gradually and whisk gently.
Pour melted butter into an 8×8-inch pan. Pour batter over melted butter.: When the melted butter hits the pan, you will hear a soft hiss and see it pool evenly; tilt the pan to coat the bottom so the batter will not stick and will bake with a rich edge. This film of butter also flavors the sauce that forms beneath the cake. If the butter is too hot it can start to cook the batter on contact, so allow it to cool until warm before pouring the batter over it.
In a separate bowl, combine the remaining 1 cup of sugar, 4 Tbsp cocoa powder and chocolate chips; sprinkle evenly over the batter.: As you drop the batter into the pan, it will spread and flatten atop the warm butter , creating an interface where the cake will rise. Smooth it gently so there are no dry pockets, and notice the contrast as the pale batter meets the glossy butter. Avoid pressing the batter into the pan, which can compact it and hinder a light texture.
Slowly pour boiling water over top of the cobbler.: The granulated sugar and extra cocoa powder form the basis of the sauce, and the mini chocolate chips will melt into little concentrated pockets, adding depth. Sprinkle evenly so the sugar will absorb the boiling water uniformly; clustering here causes uneven sauce distribution. A common slip is dumping everything to one side, which leads to inconsistent results.
Bake 40 to 45 minutes, until top of the cobbler looks set. Do not overcook or there won’t be any yummy chocolate sauce.: As the boiling water meets the sugar and cocoa, you will hear a soft simmer and see the chocolate begin to dissolve, creating a shiny, thin syrup that sinks beneath the batter and forms the signature sauce. Pour in a steady ribbon to avoid disturbing the batter layer too much, and watch the liquid seam across the surface to guarantee even coverage. If you pour too quickly you can displace the dry topping and end up with dry spots.
Bake 40 to 45 minutes, until top of the cobbler looks set: During baking you will notice the top change from pale to a light, matte brown and the edges pulling away slightly from the pan, while steam escapes and the kitchen fills with rich cocoa aromas. The surface should look set and not jiggly; underbaking will leave the cake overly wet and overbaking eliminates the luscious sauce, so check at 40 minutes and rely on visual cues rather than the clock alone. A typical mistake is leaving it too long which dries out the sauce, so remove once the top looks firm.