In a stand mixer bowl or a large mixing bowl, combine water, all-purpose flour, whole wheat flour, and sourdough discard to create the overnight sponge. Use the paddle attachment or a large spoon to mix it together.

Cover the large bowl with plastic wrap, a shower cap, or a plate and let it sit at room temperature for 6–12 hours (overnight works great).

Once the sponge is ready, it’s time to finish the batter:

Preheat your oven to 400°F (204°C).

If using coconut oil and/or melted butter, make sure it has cooled to room temperature.

Mash bananas well. Stir the spices, baking soda, salt, vanilla, and the egg into the mashed bananas.

Add the mashed banana mixture and brown sugar to the sponge. Stir well to break up the sponge and fully combine the ingredients.

Mix in the oil or butter until fully incorporated.

Tip: Make sure to mix at the bottom of the bowl, especially for large batches to make sure that all the sponge becomes infused with the banana mixture.

Optional Streusel for Sourdough Banana Muffins: In a separate bowl, use a fork to combine the soft butter and brown sugar until it becomes a crumbly, sandy texture. Set aside to top the muffins before they go into the oven.

Line a muffin tin with parchment paper cupcake liners or grease the muffin cups lightly.

Use a 1/4 cup measuring cup or a large cookie scoop to fill the cups with muffin batter. Fill each cup about 2/3 full (3/4 full will create a nice dome top).

optional: Sprinkle on a teaspoon or two of streusel on the tops of the muffins.

Place the muffins in the oven and bake for 5 minutes, then reduce the temperature to 375°F (190°C) for 15 to 18 minutes. Bake until a toothpick inserted in the center of a muffin comes out clean. The tops of the muffins should be golden brown.

Let the muffins cool for 10 minutes in the pan before transferring them to a wire rack to cool completely.