Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius). Tear off a piece of parchment paper a bit larger than the size of a 9-inch square pan and crumple it up, then use it to line the bottom and sides of the pan.

Heat a small saucepan over low heat and melt the butter, brown sugar and chocolate together until it forms a nice, smooth mixture. Remove the pan from heat and pour into a large bowl. Add the vanilla, eggs, Nutella and bananas and mix well.

Sift together the flour and cocoa powder and then mix it into the egg mixture. Fold in the chocolate chips if using and pour into the prepared pan.

Bake for 30-35 minutes until the center is set and a toothpick inserted comes out relatively clean. Remove the pan from the oven and set on top of a wire rack to cool completely.

To serve, lift up the sides of the parchment paper to remove the brownies from the pan and slice with a serrated knife.