Preheat your grill to medium. (360ºF)
Line the bottom and sides of a 8 qt. Dutch oven with heavy aluminum foil.
Spray the foil well with PAM cooking spray on sides and bottom.
Mix the water, cocoa powder, and brown sugar together and pour into the Dutch oven.
Add the bag of marshmallows, spreading them out evenly.
Cover the marshmallows with 1/2 of the prepared chocolate cake mix.
Add a layer of broken up Caramello bars.
Pour the remaining batter over the chocolate pieces.
Sprinkle chocolate chips over the cake batter.
Cover oven and bake using 8-10 briquettes bottom and 14-16 briquettes top for 60 minutes. This is approximately 360ºF on a gas grill to cook the cake.
Serve warm with whipped cream.
