Grease and flour (or line with baking paper) a loaf pan. (about 10 x 25 cm)
Preheat oven at 160° C.
Sieve flour, cocoa powder, baking powder, and baking soda in a bowl.
Mix in bwhite and brown sugar, salt and cinnamon.
Melt butter in a bowl and leave to cool a bit.
Mash banana with a fork.
Stir in buttermilk, vanilla, and eggs to melted butter.
Mix butter-mixture and mashed banana in flour-mixture. Don't overmix!
Pour in prepared loaf pan and smooth down.
Bake in preheated oven at 160° C for about 55 to 60 minutes.
Tip: If you don't have buttermilk:
Use whole fat milk and add a dash of vinegar or lemon. Leave to rest for some minutes.