Preheat oven to 350 degrees F. Butter and flour or line with parchment an 8x8 -inch baking pan.
In a medium bowl stir the chocolate chips, brown sugar, walnuts, and cinnamon together in small bowl until well blended.
In a separate bowl whisk the flour, baking soda, baking powder, and salt together.
In another large bowl beat the sugar, butter and egg together on medium speed until light and fluffy about 3 minutes.
On low speed beat in the mashed banana, buttermilk and vanilla.
Add the flour mixture all at once and beat on low speed until well combined.
Spoon just a little more than half the batter into the pan and spread it out. Sprinkle with half of the streusel.
Repeat with the remaining batter and streusel.
Bake about 45 minutes or until a toothpick in center comes out clean - melted chocolate is ok but wet batter is not.
Cool on a wire rack and cut into squares.