I found this wonderful recipe in a Penzey's catalog and it became one of my favorites. I posted the recipe as I found it, but check the notes at the bottom as I made a few suggestions for the tweaks I've made.
Preheat oven to 350. Grease a bundt or tube pan well.
In a small bown, mix the filling ingredients together well and set aside.
In a large bowl with a mixer, cream the shortening and sugar until very light and fluffy. Beat in the eggs, then the mashed banana and vanilla.
Add the sour cream and mix until well-blended.
Sift together the flour, baking powder, baking soda and salt, add this to the batter, and beat until well blended.
Sprinkle half the filling mixture into the baking pan, then top with half the batter. Repeat with the remaining filling and top with the rest of the batter.
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 5 minutes on a rack, then turn out the cake onto the rack to finish cooling.
Don't prepare the icing until the cake is completely cool and you are ready to ice it. The icing gets hard very fast so you need to use it immediately.
To make the icing, melt the 6 tablespoons of butter in a saucepan, then add the sugar and milk. Bring to the boil and continue boiling for 1 minute. Remove from the heat, add the vanilla, and gradually mix in the powdered sugar. (Note that the icing hardens rather quickly, so you need to work fast. If it hardens up too quickly for you, add some milk to thin it out.).
Pour the icing over the cooled cake and let it drip down a little over the sides.
Instead of the 1/4 cup of cinnamon sugar called for in the filling, I make my own using 3 tablespoons of sugar and 1 tablespoon of cinnamon.
The recipe calls for 1/2 cup of shortening but I will often use 1/4 cup shortening and 1/4 cup butter or margarine, which makes it a bit richer, although it is great with just shortening. I've used all butter or margarine in a pinch at times with excellent results.