Preheat oven to 180C/350F. Grease a 15cm x 25cm (6 inch x 10 inch) (top measurement) loaf pan; line base with baking paper.
Beat butter and sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time. Transfer to a large bowl; stir in sifted dry ingredients, banana, sour cream and milk. Spread mixture into the pan.
Bake the cake for about 50 minutes. Stand cake in pan for 5 minutes; transfer, top-side-up, to a wire rack to cool.
Meanwhile make passionfruit icing. Combine all ingredients in a medium bowl. Spread icing over top of cooled cake.