1.Preheat oven to 180°C/160°C fan forced. Grease and line a 19cm (base measurement) square cake pan with baking paper, extending paper at two sides for handles.
2.Using an electric mixer, beat butter, essence, sugar, eggs, flour, bicarbonate of soda, banana and 2 tablespoons of the milk in a large bowl until smooth and pale.
3.Combine cocoa powder and remaining milk in a small bowl. Add to one portion of batter. Stir to combine.
4.Spoon half the batter into a medium bowl.
5.Place alternate spoonfuls of each batter into prepared pan. Using a spoon, swirl flavours together to create a marbled effect.
6.Bake for 45 minutes or until a skewer inserted at centre comes out clean. Cool cake in pan for 5 minutes. Transfer to a wire rack to cool completely.
7.To make chocolate frosting, using an electric mixer, beat cream cheese and hazelnut spread in a medium bowl until smooth. Gradually beat in icing sugar until combined. Chill. Frost top and sides of cake. Serve.