Preheat the oven to 350F and line an 8x8 pan with parchment.

Mash the bananas until mostly smooth in a large bowl.

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

Fold in most of the walnuts and spread the batter into the pan.

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

Cool fully, then chill briefly before cutting into squares.