Preheat the oven to 400 degrees. Spray an 8 by 8-inch pan with nonstick spray.
Stir together the banana slices, brown sugar, chocolate chips, and 2 tablespoons butter in a bowl. Spread evenly into the prepared pan.
Combine the baking mix, sugar, cocoa powder, remaining 2 tablespoons butter, and 2 tablespoons milk in another bowl. Using your fingers or a fork, combine until you get a moist crumble. Spread the topping evenly over the banana mixture. Bake until golden brown, 15 to 20 minutes. Let cool.
When you are ready to serve, whip the cream with the powdered sugar. Serve the crumble topped with the whipped cream.