Line a 10 inch square cake tin with non stick baking paper and set aside. Preheat the oven to 180C.
In a freestanding mixer, beat the butter and sugar until white and creamy.
In a bowl, combine the flour, salt, baking powder and baking soda. Add this to the creamed butter and sugar mixture. Mix well.
Add the bananas, buttermilk, eggs and vanilla. Gently combine and mix for about two minutes.
Pour the batter into the prepared pan. Bake for around 1 hour or until a skewer inserted in the middle comes out clean.
Cool the cake in the tin for 15 minutes before inverting onto a wire rack to cool completely.
For the frosting, beat the butter and Nutella until combined. Add icing sugar, a cup at a time until everything is smooth. Add the condensed milk and mix until glossy.
Spread the frosting onto the cooled cake.