Whisk all of the topping ingredients together in a small bowl. Keep in the fridge until ready to use.
Preheat the oven to 350°F/175°C. Grease and line a 8.5x4.5 inch loaf pan.
Mash your bananas until smooth, set aside. You can use a fork or a potato masher to mash them.
Whisk the flour, baking soda, salt and spices together. Set aside
In your mixer with the paddle attachment, beat the butter until creamy. Add your sugars and beat on medium speed until pale and fluffy, 2-3 minutes
Add the vanilla and the egg, then beat well to incorporate.
Turn the speed to low and add your mashed bananas, your mixture might be lumpy which is fine!
Add the dry ingredients in three additions on low speed, alternating with the sour cream, starting and ending with the dry ingredients
Pour the batter into your pan and tape it on the counter to remove any air bubbles.
Sprinkle over your topping mixture
Bake for 50-60 minutes, a toothpick inserted will come out with a few crumbs. If it is browning too quickly, place some foil on top
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven't tested the recipe with this method. Just click on 'US customary' above the ingredients. Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Salt: If you don't have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Sour Cream: you can substitute full fat yoghurt for the sour cream. Storage: keep at room temperature for up to 2 days or in the fridge for 5 days. Freeze slices for up to 3 months and defrost overnight in the fridge.