Preparing for baking. Preheat the oven to 350 degrees F. Butter a 9-inch round cake pan. Flour the dish with a sprinkle of white rice flour.
Mashing the bananas. Mash the bananas with a large potato masher or a fork and set aside.
Creaming the butter and adding the liquids. Mix the butter and cane and turbinado sugars together. When they are just creamed, stop mixing. Add the eggs, vanilla extract, and sour cream to the butter and sugar. Mix in the mashed bananas.
Mixing the dry ingredients. Stir together all the dry ingredients. Break up the lumps of cocoa powder with a fork. Set aside.
Combining everything. Using a rubber spatula, fold the dry ingredients into the wet batter, 1/4 cup at a time, until everything is just mixed together.
Baking the bread. Scrape the dough into your pan. Top with the demerara sugar, covering the surface of the dough. Place into the oven and bake for about 40 minutes, or until a knife inserted gently into the center of the bread comes out clean.
Cooling and serving. Let the bread sit in the pan for 5 to 10 minutes, then turn it over onto a wire rack. Serve warm, with cream cheese, if you wish.