Prepare 2 loaf pans by coating the inside of each generously with butter (I do this even if my loaf pans are non stick). See notes for pan sizes.
Cream the butter, sugar and eggs until light in color and nice and fluffy. It's best to use a stand mixer or hand mixer for steps 3 and 4 - you'll get the best result from your zucchini bread.
Now add sourdough starter and vanilla to the creamed butter, sugar and eggs and beat until well combined.
Now add all the dry ingredients and shredded zucchini (make sure you've squeezed the moisture out) into a separate bowl and mix until well combined and the zucchini is coated in flour.
Now add the wet ingredients into the bowl with the dry ingredients and fold gently until combined - try not to over mix it!
Now pour the mixture into two loaf pans that have been well greased with butter (I put 630g of mixture into each tin).
Bake the sourdough zucchini bread for around 50 minutes at 350ºF (175ºC) (the time really depends on your oven). Insert a skewer into the middle of the bread - if it comes out clean, they're ready to come out of the oven.
Allow the sourdough zucchini bread to cool in the tins for 10 to 15 minutes before removing them and placing on a wire rack to cool.
Notes
Loaf Pans - I've used 2 of these USA Pan 1 Pound Loaf Pans measuring 8.5 x 4.5 x 2.75. Zucchini - I've used one large zucchini or 2 medium zucchini when making this recipe. This equated to about 600g of shredded zucchini before removing the moisture and 400g after squeezing out all of the excess moisture. I've found that if you add a pinch of salt to the shredded zucchini and let it sit for around 30 minutes, you'll be able to squeeze out the most moisture possible.