Preheat oven to 350F degrees and adjust oven rack to the middle position. Lightly grease and flour an 8-Inch baking dish.
In the bowl of a food processor, add the chocolate chips, brown sugar, walnuts, and melted butter. Pulse until the mix is just combined and coarse. Set aside.
In a medium mixing bowl, sift the flour, baking powder, baking soda and salt.
In the bowl of a stand mixer, add the sugar, room temperature butter, and beat on medium speed until fluffy. Add in the milk, sour cream and bananas and continue to beat until all ingredients are combined. The batter will be a little lumpy because of the bananas.
Slowly add the dry ingredients to the wet ingredients on medium-low speed, until just combined. Turn off the mixer and finish incorporating the ingredients by using a spatula - scraping down the sides of the bowl.
Pour half of the batter into the prepared baking dish. Then sprinkle half of the streusel on top of the batter. Pour the remaining batter and top and very gently spread out with a spatula. Top with the remaining streusel.
Bake for 40-45 minutes, or until a toothpick inserted in the middle comes out clean. Cool on a wire rack. The cake can be served directly from the baking dish - no need to transfer to another platter.