01 - Preheat oven to 350°F. Grease a 9-inch square or round baking dish with softened butter.
02 - Gently toss sliced bananas with granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, cornstarch, and vanilla extract until evenly coated. Spread the mixture into the prepared baking dish.
03 - Whisk together flour, sugar, baking powder, and salt in a separate bowl. Pour in melted butter and milk, stirring until just combined into a thick, pourable batter. Avoid overmixing.
04 - Spoon the batter over the banana filling, spreading gently to cover most of the fruit. Sprinkle with coarse sugar if a crunchy top is desired.
05 - Bake for 35 to 40 minutes until the topping is golden brown and the filling is bubbling around the edges.
06 - Let cool for at least 10 minutes before serving warm. Pair with vanilla ice cream or whipped cream.