Preheat oven to 150C. Melt butter and chocolate in a heatproof bowl over a saucepan of simmering water, stirring occasionally until smooth. Whisk eggs and caster sugar in an electric mixer until thick and pale (5-6 minutes), fold through chocolate mixture, then fold in flour and macadamia. Spoon into a buttered and baking paper base-lined 22cm-diameter springform pan, smooth top and bake until just firm to touch (20-25 minutes). Cool completely in pan.
Heat brown sugar in a large frying pan over medium-high heat until melted (2-3 minutes). Add ginger, stir to combine, then add bananas in a single layer and cook, turning occasionally, until caramelised and tender (4-5 minutes). Arrange on top of the brownie, squash to make a uniform layer then cool completely.
Meanwhile, for milk chocolate mousse, whisk eggs and sugar in a heatproof bowl over a saucepan of simmering water until thick and can hold a ribbon (10-12 minutes), then remove from heat. Squeeze excess liquid from gelatine and whisk in gelatine with eggwhite powder. In a separate bowl, whisk cream until soft peaks form, add chocolate and fold to combine, then fold in egg mixture. Spoon mousse over caramelised banana, smooth top and refrigerate until set (2-3 hours). Dust with Dutch-process cocoa, scatter with chocolate balls and serve immediately.
Note Valrhona balls are available from Simon Johnson and specialty food stores. Eggwhite powder is a professional pâtisserie product. Pavlova Magic can be used as a substitute.