Preheat oven to 350 degrees F. Grease a 9"x5" nonstick loaf pan with vegetable spray.
Place the flour, cocoa, baking powder, salt and cinnamon in a bowl and whisk to combine. Stir in the walnuts and cranberries.
Place the butter, sugar and banana in the bowl of your stand mixer. Beat until well combined and the banana is mashed. Beat in the eggs. Mix in the buttermilk and vanilla. Add the dry ingredients to the wet and mix until just combined.
Spoon and spread the batter into the loaf pan. Bake 55 to 60 minutes, or until a toothpick inserted into the centre of the loaf pulls out clean. Cool in the pan on a baking rack for 15 minutes before unmoulding. Let cool to room temperature, and then tightly wrap loaf and keep at room temperature until ready to serve.