Preheat the oven to 350f / 180c. Butter an 8x8 inch square baking pan.

In a small bowl, combine chocolate chips, brown sugar, walnuts, and cinnamon; set aside.

In a medium bowl, sift together flour, baking soda, baking powder and salt.

In a third bowl, large this time, or the bowl of an electric mixer, cream together butter, sugar, and egg until light and fluffy. Add in the bananas and buttermilk and beat again until well combined.  Add the dry ingredients to the wet, I did mine in 2 stages and mix until well combined. Do be careful not to overmix at this stage as you want the finished cake to be soft and moist, overmixing makes for tough cake! 

Spread half of the batter on the bottom of the cake pan. Sprinkle half of the chocolate and walnut mixture over the batter. Spread the remaining half of the batter out into the pan and sprinkle with the remaining 'sprinkle'. 

Bake for 40-45 minutes until a tester comes out clean. I started checking mine at 35 minutes and I suggest you do too!