Put the dry ingredients into a mixing bowl - flour, sugar, baking powder and butter (treat butter as 'dry' for our purposes). Mix until you have an even sandy consistency.
Measure out the milk and vanilla in a jug and whisk well. Drizzle into the flour whilst mixing, until you have a smooth batter/mixture. Add the mashed banana and mix for a couple more seconds, then stop the mixer and clean down the blade.
Spoon evenly into 12 cupcake cases and bake at 170°C for 20 minutes until they bounce back to the touch.
Cool on a wire rack and in the meantime make the icing by mixing the icing sugar, butter and cream cheese together for several minutes until very light and fluffy.
Scoop out the seeds from two of the passionfruits and add to your icing. Mix this through with a spoon until fairly even. Avoid using the mixer for this stage as you'll crush the passionfruit seeds.
Pipe the icing on top of the cakes using a reusable piping bag and large star nozzle and place a few seeds from the final passion fruit on top of each of the cakes, and serve. All your egg-free friends will fall over themselves to munch your cakes!