1.Preheat oven to 180°C (350°F). Line 16 holes of two 12-hole (80ml) muffin pans with paper cases.
2.Beat butter and sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time.
3.Transfer mixture to a large bowl; stir in sifted dry ingredients, mashed banana, sour cream and milk, in two batches. Drop ¼ cups (60ml) of mixture into paper cases.
4.Bake 30 minutes or until a skewer inserted into the centre comes out clean. Stand cakes in pans 5 minutes before turning, top-side up, onto wire racks to cool.
5.Meanwhile, beat cream in a small bowl with an electric mixer until firm peaks form.
6.Dollop generous spoonfuls of cream on top of cooled cakes; top each with 2 slices of banana, then drizzle with passionfruit.
You will need 2 large over-ripe bananas (460g) to get the required amount of mashed banana. Freeze un-iced cupcakes for up to 2 months in single layers to prevent damage; sealable plastic containers are good for this. For best results, try to fill as much of the space in the container as possible to eliminate air. Thaw at room temperature as close to decorating time as possible.