Preheat oven to 180° C. Grease a loaf tin with some butter and dust with some flour.
Cream together butter and castor sugar in a bowl using an electric beater. Add egg and whisk well.
Peel and chop bananas and add. Add vanilla essence and whisk. Add buttermilk and whisk well.
Sieve together flour, baking soda and baking powder into the banana mixture. Add cinnamon powder and salt and fold in well.
Transfer this mixture into the greased loaf tin and tap.
To make crumb, combine butter and flour in a bowl and mix well with fingertips. Add cinnamon powder and mix. Add powdered sugar and mix well with fingertips till the mixture resembles breadcrumbs.
Spread the crumb mixture on top of banana mixture. Place the tin in preheated oven and bake for 40-45 minutes. Remove from oven, cool and demould.
Slice and serve.