Mix all of the topping ingredients together and set aside.
Combine melted butter, sugar, eggs and bananas and mix well.
In a separate bowl, combine the sour cream and baking soda, then set aside.
In another bowl, combine the flour, salt, and baking powder. To the butter mixture, alternately add the flour mixture and the sour cream mixture, beginning and ending with the dry mixture. Stir in the vanilla extract.
In the prepared bundt pan, spread half of the batter, sprinkle the batter with half of the topping mix. Repeat with the remaining batter and topping.
Bake for 55 - 60 minutes, or until a cake tester comes out clean.
Cool completely on rack before removing from the baking pan. Dust with icing sugar prior to serving.