01 - Preheat oven to 350°F. Grease a 9-inch square or round baking dish with the 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 into 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. Do not overmix.
04 - Spoon the batter over the banana filling, spreading gently to cover most of the fruit. Sprinkle with coarse sugar if desired for a crunchy top.
05 - Bake for 35 to 40 minutes, until the topping is golden brown and the filling is bubbling around the edges.
06 - Cool for at least 10 minutes before serving warm, ideally with vanilla ice cream or whipped cream.