Preheat oven to 350 degrees and line a 9 x 9 circular baking pan with parchment paper and set aside. (I make sure to use enough parchment paper so that it protrudes out of the pan's borders and I can use it to pull the brownies out completely at the end.)
In a mixing bowl with paddle attachment, add banana and mix until smooth and no chunks are visible.
In a microwave safe bowl, melt butter in microwave for 45 seconds. Pour into mixing bowl. Add both sugars and beat for 1 minute. Add in instant coffee powder and mix until incorporated.
Add cocoa, baking powder, salt, and vanilla. Mix for another minute.
Add egg and nutella to mixing bowl. Blend until incorporated.
Gradually add in flour and allow the mixture to combine well until no pockets of flour are visible. Fold in chocolate chips.
Pour brownie batter into prepared baking pan. Top with chopped walnuts on top. Bake for 30 minutes, or until a knife can pierce the middle and come out cleanly.
When ready, remove from the oven and allow to cool on counter. Gently lift the brownies from the pan by using the parchment paper that is protruding out. Cut in equal wedges.
Serve with a cold scoop of vanilla ice cream on top or a freshly brewed cup of coffee on the side!