1.Preheat oven to 220°C/425°F. Grease a deep 20cm (8-inch) round cake pan; line base with baking paper.
2.Pour condensed milk into a 2-litre (8-cup) ceramic ovenproof dish. Cover dish with foil; crush excess foil upwards. Place ceramic dish in a larger baking dish; add enough boiling water into the baking dish to come halfway up side of the ceramic dish. Bake for 1 hour. Stir mixture; cover, bake a further 30 minutes or until mixture is a golden caramel colour, adding extra boiling water to baking dish as needed to maintain water level during baking. Remove ceramic dish from water. Whisk in 170g (5½ ounces) of the butter, piece by piece, until melted and caramel is smooth. Cover; refrigerate for 1 hour, stirring occasionally until mixture is cold and firm.
3.Reduce oven to 180°C/350°F.
4.Beat remaining butter and the sugar in a small bowl with an electric mixer until pale and fluffy. Beat in eggs, one at a time, until just combined. Transfer mixture to a large bowl. Stir in sifted dry ingredients with mashed banana, sour cream, nuts and combined coffee and water. Spread mixture in pan.
5.Bake cake for 1 hour or until a skewer inserted into the centre comes out clean. Stand cake in pan for 5 minutes before turning, top-side up, onto a wire rack to cool.
6.Beat cold caramel mixture in a small bowl with an electric mixer for 1 minute or until paler and fluffy. Spread caramel frosting on top of cooled cake. Just before serving, sprinkle with salt.
You will need about 4 medium bananas for the amount of mashed banana required. Do-ahead: The cake can be made a day ahead. Store in an airtight container at room temperature in a cool place.Note