Preheat oven to 350 degrees F. Line a standard 8 1/2 x 4 1/2-inch loaf pan (9x5 also works for a slightly shorter loaf) with parchment paper that hangs over the edges.
Mash the bananas in a large bowl, then mix in the sugar, coconut oil, flax egg, and vanilla.
Add the flour, baking powder, baking soda, and salt to the bowl and stir everything together, just until combined. Stir in the chocolate chips or nuts, if desired.
Pour batter into the prepared pan and bake 50-60 minutes, until the top is browned and a toothpick inserted into the center comes out clean (you may choose to cover the top with foil for the last 15 minutes of baking if it is browning too much for your liking). Allow loaf to cool for 10 minutes in the pan, then remove using the parchment overhang to cool on a wire rack before slicing.