Preheat the oven to 350 degrees and lightly grease or butter the sides of a 9×13 inch glass baking dish.
In a medium bowl, whisk the flour, whole wheat pastry flour, baking powder, salt, and cocoa.
In a large bowl, whisk together the eggs and sugar until smooth. Add in the milk and melted coconut oil, whisking until combined. Stir in mashed bananas and vanilla until mixed. Slowly stir in dry ingredients and mix until just combined (this can also be done with a stand mixer). Fold in the chocolate chips.
In a small bowl, blend the 4 tablespoons of butter with the brown sugar and spread it in the baking dish. Arrange the sliced bananas in the dish. Spoon dollops of the peanut butter into the dish and fill in the gaps with the chocolate banana cake batter. Bake for about 45-55 minutes.
Let the cake cool for 10 minutes, then loosen the edges with a knife and invert onto a platter. Let the cake cool slightly, then cut into squares and serve with the peanut butter carmel sauce.
While the cake cake cools make the peanut butter carmel sauce. Mix 4 tablespoons of butter with 1/4 cup brown sugar, 2 tablespoon peanut butter and 1 teaspoon vanilla, then melt it in the microwave for 30 seconds at a time, stirring after each interval, for a total of 1 minute. Pour the warm sauce over the cake.