Heat oven to 350°F. Line the bottom of an 8-inch cake pan with parchment, then set aside.
Heat a medium pan over medium, add butter and melt, about 1 minute, making sure it doesn’t brown. Once butter is melted, add miso and brown sugar and whisk to combine. Cook until thick and viscous, about 2 minutes.
Pour the caramel into the bottom of the cake pan and swirl it around to coat the surface evenly, using a spatula as needed. Arrange the halved bananas cut-side down.
In a medium bowl, mix the sour cream, overripe bananas, vanilla, and baking soda.
Using an electric mixer, cream the eggs with granulated sugar in a separate bowl until pale yellow. Slowly stream in the oil and continue to mix until combined.
Add the banana mixture to the creamed eggs and stir to combine. Add the flour and salt, then fold gently using a spatula to incorporate.
Pour batter over the caramel and cut bananas, then transfer to oven. Bake until cooked through, about 60 minutes.
Remove cake from oven and run a knife around the edge of the cake. Let cool for about 5 minutes, then invert onto a serving plate. If the parchment slides out with the cake, simply peel it off immediately after it’s flipped.
Let rest for at least 15 minutes before enjoying.
