01 - Preheat the oven to 350°F. Grease a 9-inch square or round baking dish with softened butter.
02 - In a large bowl, gently toss sliced bananas with granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, cornstarch, and vanilla extract until evenly coated. Spread the mixture in the prepared baking dish.
03 - In a separate bowl, whisk together flour, sugar, baking powder, and salt. Add 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. Pairs well with vanilla ice cream or whipped cream.