Preheat the oven to 350ºF. Lightly grease a 9-inch cake pan. Set aside.
Melt 3 tablespoons of butter in a small pot over medium heat. Once melted, add ½ cup brown sugar and 2 teaspoons corn syrup. Stir to combine. Cook over medium heat for 3 minutes, stirring occasionally, until the sugar dissolves and the mixture darkens slightly in color.
Pour the mixture into the prepared cake pan and smooth it into an even layer. Transfer to the refrigerator for 15 minutes.
Make the batter:
Sift ¾ cup cake flour, 1 teaspoon baking powder, and ¼ teaspoon kosher salt into a small bowl. Set aside. Add 4 tablespoons butter and ½ cup light brown sugar to a large bowl. Use an electric hand mixer to cream the butter and sugar until light and fluffy. Add 1 egg and mix until smooth.
Add half the sifted dry ingredients and beat with the mixer until just combined. Add half the milk and beat until incorporated. Add the remaining dry ingredients and the remaining milk, along with 1 teaspoon vanilla extract. Beat until just combined. Don’t over-mix!
Assemble the cake:
Peel the bananas and slice them crosswise into ½-inch rounds.
Remove the cake pan from the refrigerator. Arrange the banana rounds in a single layer. Use your hands to press them gently into the caramel layer.
Pour the crushed nuts and chocolate over the bananas.
Spoon the batter over the chocolate-nut layer and carefully smooth it into an even layer.
Bake the upside-down cake:
Transfer the cake to the oven for 35 to 40 minutes or until a toothpick inserted into the center of the cake comes out clean.
Let the cake stand for at least 15 to 30 minutes at room temperature.
To serve:
Place a large plate over the cake pan and carefully flip the cake onto the plate. Use a small butter knife to tap over the pan to encourage the release of the cake. Once the cake has been transferred to the plate, cut and serve with whipped cream on top. Enjoy!