Heat your oven to 350°F and lightly oil a 9x5 inch loaf pan or similar size, making sure to coat the bottom and sides evenly to prevent sticking. While the oven preheats, you can move on to your mise en place since this will take about 10-15 minutes to reach temperature.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon until evenly distributed. This ensures the leavening agents and spices are evenly dispersed throughout the batter, which prevents dense pockets in your finished bread. Set this mixture aside.

In a large bowl, whisk together the melted coconut oil (cooled to room temperature) and sugar until combined, then add the egg and whisk until the mixture is smooth and slightly pale. This creaming step helps incorporate air into the batter. Stir in the mashed bananas and sourdough starter until no large streaks remain, then add the vanilla extract and mix gently until just combined.

Add the dry ingredient mixture from Step 2 to the wet ingredients from Step 3, and fold together with a spatula or wooden spoon using just 15-20 strokes—stop as soon as you no longer see streaks of flour. I find that over-mixing banana bread develops the gluten too much, which creates a tough, dense crumb instead of the tender texture we're after. The batter should look slightly lumpy.

Gently fold the flour-coated blueberries (which prevent them from sinking to the bottom during baking) into the batter with just a few strokes of your spatula. Pour the batter into the prepared loaf pan and smooth the top with a spatula, making sure it's evenly distributed.

Place the pan in the preheated 350°F oven and bake for 55-65 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs (not batter). The top should be deep golden brown. If the top is browning too quickly before the center is done, tent it loosely with foil for the last 15-20 minutes. Remove from the oven and let cool in the pan for 15 minutes, then turn out onto a wire rack to cool completely before slicing.