Preheat oven to 350F; parchment-line 8x8 pan.

Mash bananas mostly smooth in large bowl.

Stir in sugar, coconut oil, and vanilla; mix in flour and cocoa.

Fold in most walnuts; spread batter in pan.

Scatter remaining walnuts on top; bake 28 to 30 minutes until set.

Cool fully; chill briefly before cutting squares.