Special equipment: a 9 1/2-by-3 1/2-inch bundt pan
For the cake: Preheat the oven to 350 degrees F. Spray a 9 1/2-by-3 1/2-inch bundt pan with nonstick cooking spray.
Prepare the quick bread mix according to the package instructions. Pour the batter into the prepared bundt pan and bake until a toothpick inserted in the cake comes out clean, 50 minutes to 1 hour. Let cool.
For the filling: Melt the butter in a large skillet over medium heat. Add the brown sugar, maple syrup, vanilla, cinnamon and salt and mix until dissolved into the butter. Remove the skillet from the heat (turn off the heat) and stir in the rum. Return the skillet to medium heat and bring the mixture to a simmer. Add the diced bananas and cook until softened just a little bit but not mushy, 1 to 2 minutes. Let cool until warm.
For the glaze and garnish: Heat the cream in a medium pot over low heat just to a simmer. Remove from the heat, add the butterscotch chips and let sit until melted, 1 to 2 minutes. Add the butter and mix until well blended. Let cool completely so that it is thick but still pourable.
Place the cake presentation-side up on a large round platter. Using a spoon, remove some of the cake from the hole in the center to make a slightly larger well. Fill the hole with the banana filling; the filling should come up to about 1/4 inch from the top surface of the cake. You should have some of the filling left over, which can be used to top the individual slices of cake or mixed into a milk shake. Starting in the center, gently pour the glaze over the filling, slowly moving in a concentric circle toward the outer part of the cake and allowing some of the glaze to pour down the sides of the cake. Garnish with the candied walnuts. Serve within 15 minutes of assembly. Slice and let the filling ooze out of the cake.