Preheat Oven: Preheat oven to 356ºF (180ºC). Grease with butter a round 8-inch baking pan and line with baking paper, set aside. NOTE: Alternatively, if you have a quality non-stick pan, liberally grease it with melted butter and dust it with a light layer of cocoa powder. This will leave your finished cake with smooth sides.
Mix Dry Ingredients: In a large mixing bowl, sift together the flour, cocoa powder, baking powder, and baking soda. Add sugar and salt and whisk until well combined.
Prepare Bananas: Peel bananas and mash in a bowl with a fork.
Combine Wet Ingredients: Add in mashed banana, milk, oil, vinegar, vanilla, and then the boiling water. Gently fold with a spatula until well combined, ensuring not to over-mix. Just ensure there is no flour remaining in the batter.
Transfer To Pan: Pour batter into the prepared baking pan, smoothing out the top.
Bake: Bake for 30-35 minutes, until a toothpick inserted into the middle comes out clean or with a few moist crumbs, not wet batter. It should spring back when lightly touched with your finger.
Cool: Remove from the oven and allow the cake to cool in the pan for 15 minutes.
Cool Completely: Transfer to a wire rack to cool completely.