Preheat the oven to 350F degrees.Add the butter and sugar to a large bowl. Microwave for 1 minute on medium power, then remove from the microwave and stir.
Whisk in the eggs and vanilla extract, followed by the mashed bananas.
Gently stir in the flour, cocoa powder, and salt.
Stir in the walnuts.
Pour the batter into the prepared pan and bake for 35-40 minutes, or until an inserted toothpick comes out clean or with a few damp crumbs.
Cool fully n then slice. Store brownies in an airtight container in the fridge for up to 4 days.
