Preheat your oven to 350°F (175°C). Grease a 9-inch round or square baking pan with butter or cooking spray.
In a small saucepan, melt 4 tablespoons of butter over medium heat. Add 1/4 cup of brown sugar and stir until combined and bubbly. Pour this mixture into the bottom of your prepared pan. Arrange the sliced banana evenly over the top of the sugar mixture. Set aside.
In a large mixing bowl, whisk together the all-purpose flour, whole wheat pastry flour, baking powder, salt, and cocoa powder.
In another bowl, beat together the sugar and eggs until light and fluffy. Add in the milk, melted coconut oil, mashed bananas, and vanilla extract, mixing until well combined.
Gradually add the dry mixture to the wet mixture, stirring until just combined. Fold in the chunky peanut butter and chocolate chips until evenly distributed.
Gently pour the batter over the banana and brown sugar mixture in the pan, spreading it out evenly.
Place the pan in the oven and bake for about 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before inverting it onto a plate. Serve warm or at room temperature.