Take butter and eggs out of the fridge about 20 minutes prior to cooking. Eggs should reach room temperature. Butter should be soft, but not melted.
Preheat oven to 350°F. Grease a 9x5-inch loaf pan with butter and set aside.
Combine slightly softened butter and sugar in a bowl and beat with mixer for about 3-4 minutes until you get smooth texture.
Add eggs one at a time and keep beating the mixture for about 1-2 minutes.
Mash bananas in a separate bowl. Add mashed bananas, sour cream and vanilla extract to the batter and beat together for a minute.
Sift flour, salt and baking powder together. Add to the batter and fold carefully using a rubber or wooden spatula. Don't use mixer at this step.
Toast walnuts on a hot pan for a few minutes. It will help to bring out the nutty flavor and keep them from getting soggy. Chop walnuts roughly, add to the batter and mix them in with a spatula.
Transfer batter from the bowl to the greased pan. Set on the middle rack of the oven and set timer for 60 minutes.
Baking time depends on the moisture content of the bananas you use. To be on the safe side, set the timer for 60 minutes first. The top of the bread should be golden brown and slightly cracked. Check readiness by inserting a toothpick in the middle, it should come out clean. Although most recipes call for 1 hour in the oven, in my experience it takes about 1 hour and 15 minutes for the bread to fully bake. If after 60 minutes the bread is not ready, leave it in the oven and check back every 5 minutes.
Take the pan out of the oven and let it cool for 10 minutes. Transfer bread from the pan to the cooling rack and leave for another 10-15 minutes. If you start slicing the bread while it's still warm, it might fall apart. But warm fresh out of the oven banana bread is the best, so if you don't care about presentation, go for it! If you let it cool completely it will be easier to slice it.