1.Preheat oven to 180°C (160°C fan forced). Line 16 holes of two 12-hole (⅓-cup/80ml) muffin pans with paper cases.
2.Beat butter and sugar in small bowl with electric mixer until light and fluffy. Beat in eggs, one at a time. Transfer mixture to large bowl, stir in sifted dry ingredients, banana, sour cream and milk, in two batches. Drop ¼ cups of mixture into paper cases.
3.Bake about 30 minutes. Stand cakes in pans 5 minutes before turning, top-side up, onto wire racks to cool.
4.To make fluffy frosting. Combine sugar and the water in small saucepan, stir over heat, without boiling, until sugar is dissolved. Boil, uncovered, without stirring, about 5 minutes or until syrup reaches 116°C (96°C fan forced) on a candy thermometer. Syrup should be thick but not coloured. Remove from heat, allow bubbles to subside. Beat egg whites in small bowl with electric mixer until soft peaks form. While motor is operating, add hot syrup in a thin steady stream, beat on high speed about 10 minutes or until mixture is thick.
5.Make fluffy frosting. Dollop cold cakes with a generous spoonful of frosting. Use a spatula to shape and swirl the “snow”. Sprinkle cakes with sugar snowflakes.
You need two large (460g) overripe bananas to get the amount of mashed banana. If you don’t have a candy thermometer, boil the syrup until it’s thick with heavy bubbles. Remove from heat, let bubbles subside, then reassess the thickness of the syrup.