Preheat oven to 350 degrees (F)
Grease a 9 x 5 x 3 in loaf pan, set aside
Whisk together whole wheat flour, all purpose flour, flax seed, salt, baking powder, baking soda, and cinnamon in a bowl
Add eggs, butter, sugar, vanilla, and bananas to a bowl and mix until combined
Add the flour mixture to the wet ingredients and stir until just combined
Stir in walnuts
Pour batter into pan, sprinkle with additional chopped walnuts if desired. Bake until golden or a toothpick inserted into the middle comes out clean, about 35 minutes
Let cool slightly before transferring to a wire rack to cool completely