Melt 2 tablespoons of the butter in a large skillet over medium-low heat and add the onions. When the onions begin to soften, add the sugar and season with salt and pepper. Turn the heat to low and cover the pan. When the onions just begin to brown, remove the cover. Continue cooking, stirring frequently, until the onions are a dark brown color. Be careful not to let them burn. Set aside.
Melt 2 more tablespoons butter in same skillet over medium heat. Cut several shallow slashes across the hot dogs and add them to the pan. Fry until they are cooked through and nicely browned. Set aside and keep warm. Add the cocktail franks to the pan and fry until they are cooked through and nicely browned. Set aside and keep warm.
Melt 2 tablespoons butter in another pan and set aside.
Add the remaining 2 tablespoons butter to the skillet. Split open the hot dog buns and toast them in the buttery juices. Spread a thin layer of mousse inside both halves of each bun. Top with a hot dog and pile on the onions. Add a few dollops of mustard and serve.
Split the dinner rolls down the middle and spoon on some of the melted butter. Stuff in 2 of the cocktail franks and spread on some mousse. Top with some onions and mustard and serve.