Add the eggs one at a time, beating well after each addition to fully incorporate. Scrape down the sides of the bowl as needed to keep the mixture smooth and even.
Next, stir in the mashed bananas and vanilla extract. The mixture may look slightly curdled or uneven at this stage, especially because of the moisture from the bananas. That is completely normal and will smooth out once the flour is added.
Step 5: Combine Wet and Dry Ingredients
Add the flour mixture in three additions, alternating with the milk or sour cream. Begin with flour, then add half the milk, followed by more flour, the remaining milk, and finish with the final portion of flour.
Mix gently after each addition, stopping as soon as the flour disappears. Overmixing at this stage can develop too much gluten, resulting in dense or tough banana bread. The batter should look smooth but not overly beaten.
Step 6: Fold in Nuts (Optional)
Using a spatula, gently fold in the chopped walnuts until evenly distributed throughout the batter.
Avoid stirring too aggressively. Gentle folding keeps the batter airy while adding a pleasant crunch and contrast to the soft crumb.
Step 7: Bake Until Done
Pour the batter into the prepared loaf pan and smooth the top evenly with a spatula. Tap the pan lightly on the counter to release any large air pockets.
Bake for 55–65 minutes, checking at the 55-minute mark. Insert a toothpick into the center; it should come out clean or with a few moist crumbs, but no wet batter. If the top begins to brown too quickly before the center is fully baked, tent the loaf loosely with aluminum foil during the final 10–15 minutes.
Step 8: Cool Before Slicing
Remove the pan from the oven and allow the banana bread to cool in the pan for 10–15 minutes. This short resting time helps the structure firm up slightly.
Carefully lift the loaf out using the parchment overhang and transfer it to a wire rack to cool completely. Allowing it to cool fully ensures clean slices and helps the flavors develop more fully.