Preheat the oven to 175 degrees. Grease a baking sheet with a bread pan.
Mash the banana in a bowl to a puree consistency, you should make about 1/2 cup.
In a large bowl, combine flour, sugar, baking powder, baking soda and 1/2 teaspoon salt. Add melted butter and buttermilk. Add egg and beat for 1 minute.
Add banana puree and vanilla extract. Stir well.
Put the resulting dough into the prepared bread pan, smooth the surface with a rubber spatula.
To make a sprinkling for the top, mix chilled butter, diced, with flour, brown sugar, cinnamon and a pinch of salt.
Sprinkle the mixture over the top of the bread. Bake the bread in a preheated oven for 40-45 minutes, until a toothpick inserted in the center comes out clean.
Allow to cool on a wire shelf for 10 minutes. Cut the bread into slices and serve.