Using the same bowl (no need to clean it), whisk together the cocoa powder, brown sugar, water, and espresso powder, if using.
Pour the chocolate fudge mixture over the batter in the pan, but do not mix them together. Sprinkle with the chocolate chips.
Bake, rotating once partway through baking, until the cake still looks slightly soft and underbaked in the middle, but drier and cakelike around the edges, and when you shimmy the pan on the oven rack (with a potholder-clad hand!), the middle of the cake jiggles, 30 to 40 minutes.
Remove the cake from the oven. Let rest at least 15 minutes if you want a fudgy, sauce-like bottom layer, and 30 minutes for a more pudding-like experience. (Both are spectacular, FYI.)
Serve warm, straight from the pan with a drizzle of heavy cream or a scoop of vanilla ice cream. To keep the cake, wrap and leave on the counter, for up to 3 days.