Preheat the oven at 160c. Line a 25 x 11cm bread pan with butter or greaseproof paper.
Mash the bananas in a bowl and set aside.
Cream the butter, sugar and eggs in a large bowl. Add the mashed banana in the butter mixture until everything is mixed well.
Add the flour one tablespoon at a time. Don't worry if the mixture starts to go lumpy. Keep adding the flour until all the ingredients are incorporated. Add the milk at the end and mix well.
Place the nutella in a jug and heat in the microwave for 30 seconds or until the chocolate is easy to pour.
Pour half of the banana cake mixture in the pan. Add ¼ of the Nutella in the middle to make the filling. Fill the rest of the pan with the cake mixture then use the rest of the Nutella to decorate to the top.
Bake for 50 - 60 minutes, or until a toothpick inserted into the centre comes out clean. Cover the tin loosely with foil halfway through baking to prevent the bread from burning. Leave to cool completely in the pan on a wire rack.