Preheat the oven to 350 degrees Fahrenheit (175 degrees C) Grease a 9 X 5 loaf pan.
In a large mixing bowl, cream butter and sugar together until fluffy, about 2 minutes. Mix in eggs, one egg at a time until fully incorporated. Add in vanilla and lemon juice.
In a separate bowl, mix flour, baking powder, baking soda and salt together. Slowly add in dry ingredients to wet ingredients, making sure to only mix until incorpated. Do not overmix.
Fold in sour cream, mashed bananas, and walnuts. Do not overmix. Pour into greased pan and bake at 350 degrees Fahrenheit for 60 minutes, or until a toothpick inserted into the middle comes out clean. Check the banana bread before 60 minutes to be sure. Do not overbake. Let bread cool in the pan for 10 minutes before transferring to a wire cooling rack.