Lightly grease a small baking dish with coconut oil or avocado oil.
In a mixing bowl, mash the banana until mostly smooth. Add the egg, almond milk, Greek yogurt, maple syrup, and vanilla extract. Whisk until fully combined.
Pour the batter into the prepared baking dish and smooth the top with a spoon or spatula.
Sprinkle the mini dark chocolate chips evenly over the surface. Add chopped walnuts if desired.
Bake for 22–25 minutes, or until the edges are set and the center remains slightly soft for a brownie-like texture.Remove from the oven and allow to cool for 5 minutes.
Serve warm topped with fresh berries, sliced banana, or an extra spoonful of Greek yogurt if desired.