1. Preheat the oven to 325 degrees.
2. Toast the walnuts on a baking sheet in the preheated oven for 7 minutes. Remove the nuts from the oven and set aside to cool at room temperature until needed.
3. In a sifter combine the flour, baking powder, and salt. Sift onto a large piece of wax paper and set aside until needed.
4. Place the butter and 1 cup sugar in the bowl of an electric mixer fitted with a paddle. Beat on medium speed for 2 minutes until soft. Use a rubber spatula to scrape down the sides of the bowl. Add the eggs, one at a time, beating on medium for 1 minute and scraping down the sides of the bowl after each addition. Add 1 teaspoon vanilla extract and beat on high for 1 minute. Add the whole peeled bananas (you can break them in half, but it really isn't necessary) and beat for 2 minutes on medium until fairly smooth. Scrape down the sides of the bowl. Operate the mixer on low while gradually adding the sifted dry ingredients until incorporate, about 1 minute. Add the chocolate chips and the toasted walnuts and mix on low to incorporate, about 20 seconds. Remove the bowl from the mixer and use a rubber spatula to finish mixing the ingredients until thoroughly combined.
5. pour the batter into the 9x13x2-inch nonstick baking pan. Use a rubber spatula to spread the batter in an even layer. Bake on the center rack of the preheated oven form for 40 minutes until golden brown around the edges and set in the center. Remove the pan from the oven and allow to stand at room temperature for 1 hour before making the icing.
6. Heat the heavy cream and 4 tablespoons sugar in a 1-1/2 quart saucepan over medium-high heat. When hot, stir to dissolve the sugar. Bring to a boil. Place the semisweet chocolate and unsweetened chocolate in a 3-quart bowl. Pour the boiling cream over the chocolate and stir with a whisk until smooth.
7. Pour the icing over the cooled Monkey Bar. Use a cake spatula to spread the icing evenly over the entire surface of the bar. Refrigerate for 1 hour before serving.
8. Use a serrated knife with rounded tip to cut the bars into 24 squares, then put each square in half to yield 48 bars. For a clean cut, heat the blade of the knife under hot running water and wipe the blade dry before making each cut.