Preheat oven to 350°F (175°C). Line the bottom of an 8-inch loaf pan with parchment paper and lightly grease the sides with a little of the coconut oil (or other oil you have).
In a medium bowl, whisk together the dry ingredients: 3 cups (324 g) oat flour, 1/4 cup (30 g) dairy-free vanilla protein powder, 1 1/2 teaspoons vegan xanthan gum, 4 teaspoons cinnamon, 6 teaspoons baking powder, and 3/4 teaspoon salt. Make sure there are no large clumps.
In a large bowl, add 2 1/4 cups (540 g) mashed ripe banana and 1 cup coconut sugar. Whisk until the sugar is mostly incorporated and the mixture is smooth.
To the banana mixture, whisk in 1/2 cup unsweetened vanilla almond milk and 2 teaspoons vanilla until combined.
Add 3 tablespoons melted coconut oil and 6 tablespoons water to the wet mixture and whisk until evenly combined.
Pour the dry ingredients into the wet mixture. Stir gently with a spatula or whisk until just combined and no large streaks of flour remain—do not overmix.
Transfer the batter to the prepared loaf pan and smooth the top with a spatula.
Bake at 350°F (175°C) for about 60–75 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. If the top is browning too quickly, loosely tent with foil for the last 15–20 minutes.
Remove the loaf from the oven and let it cool in the pan for about 15 minutes. Lift the loaf out using the parchment, then let it cool completely on a wire rack (or a flat surface) before slicing.
Slice gently with a serrated knife—the loaf is delicate.