1.Turn on the oven to 180°C/350°F and let it heat up. Spray a 14cm x 21cm (5½-inch x 8½-inch) loaf pan with cooking oil; use baking paper to line the base of the pan.
2.Put the butter and sugar in a small bowl; beat with an electric mixer until the mixture is light and fluffy (page 11). Add one egg; beat until it is mixed in well. Add the other egg; beat until it is mixed in well. Transfer the mixture to a large bowl. Use a flour sifter to sift the flour, bicarbonate of soda and mixed spice into the bowl. Add the banana, sour cream and milk; stir with a wooden spoon until the ingredients are mixed together well.
3.Put the mixture into the loaf pan; use a spatula to spread it evenly over the base. Put the pan in the oven and bake about 1¼ hours. If the cake is getting too brown in the oven; cover the pan loosely with a piece of foil.
4.Take the pan out of the oven; leave it for 5 minutes. Turn the cake from the pan onto a wire rack, top-side up; leave it to cool down.
5.Meanwhile, make the passionfruit frosting. Put the butter in a small bowl; beat with an electric mixer until it is light and fluffy. Add the cream cheese; beat until smooth. Add the sifted icing sugar; beat it into the butter. Stir in the passionfruit pulp.
6.When the cake is cold, spread the frosting over the top.
You can store the cake in an airtight container, in the fridge, for up to 4 days.Note