Mash the overripe bananas with the back of a fork until they are fairly smooth (a few lumps is fine).
Now lightly beat the eggs and vanilla together and add that to the mashed bananas. Use your fork to lightly mix the eggs through.
Melt the butter and then add that the egg and banana mixture.
Now add the sourdough starter to the egg mixture and whisk throughly.
In a separate bowl, mix all dry ingredients together and ensure that everything is mixed thoroughly.
Now add the dry ingredients to the wet ingredients and stir until just combined.
Pour mixture into a buttered loaf tin.
Bake at 180ºC/350ºF for around 1 hour. Baking time will depend on the size of your tin and your oven.
The banana bread is cooked when a skewer is inserted and comes out clean.
When you take the banana bread out of the oven, brush the top with golden syrup for a lovely shiny, sweet glaze.
Allow to cool for 30 minutes before removing from the tin.
Video
Notes
Recipe Update - the original published recipe included just 1 egg but I have since updated the recipe to include 2 eggs as I find that the finished sourdough banana bread has a better texture with 2 eggs. Vegan Substitutes - I have recently tested this sourdough banana bread using the following replacements and it's worked really well!Egg Replacement - Replace 2 eggs with 45 grams of chia seeds soaked in 125 grams of water. Let the chia seeds soak for 10 minutes before you add them to the mashed banana. Proceed with the recipe as per normal.Butter Replacement - Replace butter with 80 grams of melted coconut oil. If you're using chia seeds, add the coconut oil at the same time. Proceed with the recipe as per normal.I've tested this recipe with just replacing the eggs with chia seeds and still using butter, as well as using both chia seeds and coconut oil and both options work really well and give a delicious result!