Preheat the oven to 350 degrees.
Brush a 9-inch non-stick cake pan with butter.
In a small bowl melt 4 tablespoons of butter in the microwave. Stir in 1/2 cup of brown sugar and then pour into the bottom of the cake pan.
Spread evenly with a silicone spatula until the sugar-butter mixture forms a nice even layer.
Evenly distribute sliced bananas (I used a circular pattern) over the butter mixture.
Meanwhile in a medium bowl add the mashed banana. Stir in the egg, buttermilk, sugar, vanilla, and butter. Mix until thoroughly combined.
Gradually stir in flour, baking powder, and salt.
Finally, fold in the chocolate chips.
Pour into the cake pan over the sliced bananas.
Bake for 40-45 minutes or until toothpick inserted into the center of cake comes out clean.
Cool until cake can be handled with bare hands, then flip out of the cake pan and onto a serving plate.
Cake is best served warm.