Start by heating your oven to 350°F. Grab a 9x5-inch loaf pan and give it a light coat with nonstick cooking spray.
In a large mixing bowl, use a stand mixer or a hand mixer to cream together the softened butter and coconut sugar until it's light.
Crack in the eggs, add the mashed bananas, buttermilk, and vanilla extract. Mix until everything is well combined.
Time for the dry team! Sift in the whole wheat flour, baking powder, sea salt, and baking soda. Mix until just combined to keep the bread tender. Do not overmix.
Pour the batter into your prepared loaf pan and smooth the top with a spatula. Pop it in the oven and bake for about 50-55 minutes. You'll know it's done when a toothpick inserted into the center comes out almost clean.
Let the bread cool in the pan on a wire rack for about 15 minutes. Then, run a knife around the edges and gently remove the bread from the pan. Let it cool completely on the rack before slicing.