Preheat oven:
Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
Mix dry ingredients:
In a medium bowl, whisk together flour, baking soda, and salt.
Cream butter and sugar:
In a large bowl, beat butter (or oil) and brown sugar until smooth and creamy.
Add wet ingredients:
Mix in eggs one at a time, then add mashed bananas, vanilla extract, and sour cream. Stir until fully combined.
Combine wet and dry:
Gradually add the dry ingredients into the wet mixture. Stir gently until just combined — avoid overmixing.
Add mix-ins (optional):
Fold in walnuts, chocolate chips, or cinnamon if desired.
Bake:
Pour batter into the loaf pan and bake for 55–65 minutes, or until a toothpick inserted in the center comes out clean.
Cool and serve:
Let the banana bread cool in the pan for 10 minutes before transferring it to a wire rack. Slice and serve warm or at room temperature.