Place butter in a small bowl and cover with a napkin to avoid splatter. Microwave for 30 to 45 seconds.
Crack eggs into a medium-sized bowl. Whisk vanilla into eggs until fluffy. Add melted butter, and whisk to combine.
In a large bowl, whisk flour, sugar, baking soda, baking powder, salt, and cinnamon. If adding walnuts, chop and stir into the flour mixture.
Stir the wet ingredients into the dry ingredients until combined. Fold the mashed bananas and blueberries or huckleberries into the batter.
Grease and flour a loaf pan, then pour batter into it. Bake loaf for 45 minutes then turn the temperature down to 325 and loosely cover with aluminum foil to avoid over-browning.
Insert a toothpick into the center of the loaf. If done, the toothpick should come out clean. Insert a toothpick into the center of the loaf. If the toothpick is covered with wet batter, the banana bread needs to continue to cook. Check every 5 minutes until a toothpick comes out clean.
Cool on the counter for approximately 15 minutes then carefully invert the loaf onto a wire rack. Cool completely before slicing.