1.Preheat the oven to 180°C (160°C fan-forced). Grease a 20cm baba cake pan or a deep 20cm round cake pan.
2.Beat the butter and sugar in a small bowl with an electric mixer until fluffy. Add the egg and beat until combined. Stir in the sifted dry ingredients then the banana and milk. Spread the mixture into the prepared pan. Bake in a moderate oven for about 50 minutes. Turn onto a wire rack to cool.
3.For the passionfruit icing, sift icing sugar into a heatproof bowl. Stir in the butter and passionfruit to form a firm paste. Stir icing over pan of simmering water (or microwave on high for about 20 seconds) until the icing is a pouring consistency.
4.Spoon passionfruit icing over the cooled cake; allow to set.