Preheat oven to 350°F. Melt butter in the baking dish in the oven, about 5 to 7 minutes. Remove once fully melted.
Whisk flour, 3/4 cup sugar, baking powder, salt, and cinnamon. Add milk and vanilla and whisk until smooth.
Pour batter over melted butter. Do not stir.
Arrange banana slices over the batter. Do not stir or press down.
Sprinkle remaining granulated sugar and brown sugar evenly over the top.
Bake 45 to 55 minutes until deep golden and set in the center. Rest 10 minutes before serving warm.