Preheat oven to 350 F.
Prepare a loaf pan (9 x 5 inch). Line with parchment paper with some extra parchment paper hanging on the sides. This makes it easier to LIFT the banana bread out of the pan easily.
Take 1/4 cup walnuts, sprinkle 1 teaspoon of all purpose flour over them to coat them. Keep aside (this prevents them from sinking to the bottom).
In a bowl, mash the bananas with a fork and keep it aside. It is ok if there are a few chunks.
In another bowl, SIFT the dry ingredients- all purpose flour, baking powder, baking soda, cinnamon, salt, cardamom powder and nutmeg powder (if using). Use a WHISK to mix them. This allows the ingredients to evenly distribute.
In another larger bowl prepare the liquid batter by adding oil, brown sugar and sugar and mix until well combined. This can be done using a stand mixer or hand beater and may take around 1-2 minutes.
Gradually add 1 egg at a time, making sure to mix well after each addition. Continue to mix for 1-2 minutes. The batter will start to look light and fluffy.
Then add in the yogurt (or sour cream) and vanilla and give one final mix.
From here onwards, use a spatula (we do not want to liquify the bananas or overmix the dry ingredients).
Add the mashed bananas to the liquid batter and mix with the spatula.
Then in 2-3 batches, gradually add in the dry ingredients and FOLD with each addition. Do not overmix! Stop folding when you no longer see any dry ingredients (a few streaks are ok).
Add in the 1/4 cup of the walnuts we had coated in Step 3 and fold it in the batter gently.
Pour the banana batter into the loaf pan and scatter the remaining 1/4 cup of walnuts on the top.
Bake in the preheated oven for 55-60 minutes.
Check for doneness around the 55 minute mark. The toothpick should come out with a few crumbs on it. If it comes out wet, give it an extra 5 minutes.
Once done, remove the loaf pan from the oven and place it on the cooling rack for 10-15 minutes.
Then gently lift out the banana bread (using the overhanging parchment paper) and allow it to cool completely on the cooling rack.