1.Preheat oven to 170°C. Grease a deep 14cm x 21cm loaf pan; line base and sides with baking paper, extending the paper 5cm over sides.
2.Using a vegetable peeler, remove rind from orange. Reserve rind. Using a small sharp knife, remove white pith from orange; discard pith. Quarter orange and discard seeds. Blend or process orange flesh and rind until pulpy. Stir in passionfruit pulp.
3.Beat butter, sugar, eggs, sifted flour, milk and orange mixture in a large bowl with an electric mixer on low speed until combined. Increase speed to high and beat about 2 minutes or until paler in colour. Spread mixture into pan.
4.Bake cake about 1 hour 10 minutes. Stand in pan 10 minutes; turn, top-side up, onto a wire rack over an oven tray to cool.
5.Meanwhile, to make passionfruit icing, stir ingredients in a medium bowl until mixture is smooth.
6.Spread icing over cooled cake.