Lightly butter and flour pan a mini loaf pan (6×3 inches or a pan that fits 2 cups liquid). Heat the oven to 325°F.
Make the chocolate batter: combine the flour, sugar, cocoa, baking soda, and salt in a bowl. Make a well in the centre and pour in the water, oil, vinegar, and vanilla. Whisk until blended. The batter will still be lumpy. Pour into the prepared pan and set aside while you make the banana top.
Make the banana batter: combine the flour, baking soda, cinnamon and salt in a bowl and set aside. Beat together the sugar and eggs until light and fluffy. Slowly drizzle in the oil while whisking, taking your time. Stir in the mashed banana, sour cream, and vanilla until just combined. Use a rubber spatula to fold in the flour mixture until thoroughly combined. No flour streaks should be visible.
Pour the batter on top of the chocolate layer and bake for 30-40 minutes. The cake should be golden brown on top and the cake will spring back when you gently press it and a skewer in the middle will come out clean. Cover with foil if it starts to brown too much.
Let cool in the pan on a wire rack for 30 minutes, remove from the pan and let cool completely on the wire rack. Slice and enjoy!