Heat oven to 350°F. Grease an 8-inch square baking dish.
Whisk flour, granulated sugar, cocoa, baking powder, salt, and espresso powder in a bowl.
Stir in milk, melted butter, and vanilla until a smooth, thick batter forms.
Spread the batter into the pan and scatter chocolate chips over the top.
Combine brown sugar, remaining granulated sugar, and cocoa. Sprinkle evenly over the batter.
Pour very hot water over the back of a spoon to cover the surface. Do not stir.
Bake 30–35 minutes, until the top looks set and sauce bubbles around the edges.
Cool 10 minutes to let the fudge sauce thicken. Scoop and serve warm.
Notes
Swap the hot water for hot coffee to boost the chocolate flavor. Serve with vanilla ice cream or a dollop of whipped cream. Refrigerate leftovers up to 3 days; rewarm gently to revive the sauce.This recipe is an original creation inspired by classic Easy Hot Fudge Pudding Cake flavors. All ingredient ratios and instructions are independently developed.