Preheat your oven to 375°F (190°C) and spray a 12-cup muffin tin with baking spray. Set aside.
Take a large bowl and mix together 1 cup flour, ¼ cup cornstarch, ¼ cup sugar, and 1 teaspoon baking powder. Whisk well to remove any lumps.
In another bowl, combine 1 egg, ½ cup milk, and ¼ cup melted butter – make sure these are all at room temperature. Whisk until well mixed.
Pour your wet ingredients into the dry ingredients. Mix gently with a spatula just until combined. Some small flour streaks are okay – don't overmix.
Cut 4 ripe saba bananas into small cubes. Fold them gently into your batter.
Divide the batter evenly between the 12 muffin cups.
Bake at 375°F for 17 minutes until just cooked through.
Remove from oven and turn on your broiler to 500°F.
While the broiler heats, brush the tops of your Maruya generously with ¼ cup melted butter, then sprinkle with ¼ cup sugar.
Place under the broiler for 1-2 minutes, watching carefully, until the tops turn golden brown and crispy.
Take out of the oven and let rest in the pan for 5 minutes.
Transfer to a wire rack to cool slightly for another 5 minutes.
Serve warm, optionally with a drizzle of coco jam or a cup of hot chocolate or ginger tea.