Preheat oven to 350 degrees. In the bowl of an electric mixer, add butter and sugar. Cream together on medium speed until light and fluffy, about 3-4 minutes. Add eggs and bananas to mixing bowl. Mix on medium speed until combined. TIP: I like to leave some larger pieces of banana in my batter so that they're visible and provide a punch of banana to the finished bread. Add flour, baking soda, salt and vanilla to mixing bowl. Mix on low speed until just combined. TIP: You want to mix only until the separate ingredients are barely visible as you'll continue to stir to mix in the walnuts and blueberries in the next step. If you over mix the batter, it will result in a more dense and chewy bread. Add walnuts and blueberries to mixing bowl. With a spatula, fold in walnuts and blueberries until just combined. TIP: Folding is another name for very gently stirring that's typically done with a rubber spatula until ingredients are just incorporated. The goal is to avoid deflating delicate ingredients by making broad, scoop-like movements from the inside of the bowl outwards. The more you handle the batter gently, the more tender your bread will be! Pour batter in a greased and parchment lined loaf pan and bake at 350 degrees for 55 minutes. Turn oven off and leave bread in oven for additional 15 minutes.