Preheat your oven to 180°C (350°F). While it heats, gather all ingredients and allow eggs to reach room temperature if needed—this helps them incorporate smoothly into the batter. Grease a standard loaf tin (about 9x5 inches) with butter or cooking spray, then line the bottom with parchment paper for easy removal. Sift the baking powder into a small bowl to remove any lumps, which ensures even leavening throughout the bread.

Mash the very overripe bananas in a large mixing bowl until mostly smooth with just a few small lumps remaining—this creates the best texture. Add the room-temperature eggs and vanilla extract, whisking vigorously until well combined. Melt the butter over gentle heat or in the microwave, then stir it into the banana mixture along with the sourdough starter. Mix until everything is fully incorporated and smooth. I find that sourdough starter incorporates best when gently folded in rather than whisked aggressively, which helps preserve some of the tangy flavor.

In a separate bowl, whisk together the all-purpose flour, sifted baking powder, brown sugar, granulated sugar, salt, cinnamon, and nutmeg. Make sure everything is evenly distributed, especially the spices, so each bite has consistent flavor throughout the bread.

Pour the dry ingredient mixture from Step 3 into the wet ingredients from Step 2, folding gently with a spatula until just combined. Don't overmix—a few small flour streaks are fine, and overmixing develops gluten which can make the bread tough and dense. The batter should be thick but pourable.

Pour the batter into your prepared loaf tin and smooth the top with a spatula. Bake at 180°C (350°F) for approximately 1 hour, or until a toothpick inserted into the center comes out clean with just a few moist crumbs—not wet batter. The bread is done when the top is golden brown and the edges pull slightly away from the sides of the tin.

Remove the bread from the oven and immediately brush the top with the golden syrup while still warm—the heat helps the syrup soak in slightly for extra moisture and richness. Let the bread cool in the tin for 30 minutes, which allows it to set enough to handle without falling apart. I always brush with syrup right out of the oven rather than waiting, as it gives a better finish and helps lock in moisture.

Turn the banana bread out onto a wire rack and peel away the parchment paper if using. Allow it to cool completely before slicing—this rests the crumb structure and makes for cleaner, neater slices. Once cooled, wrap tightly and store at room temperature for up to 3 days, or freeze for longer storage.