Beat together the butter and sugar in a bowl until very pale and creamy. Gradually beat in the eggs. Add the vanilla and buttermilk, then sift in the flour and baking powder. Gently stir together until just combined.
Spoon the mixture into a greased and floured 1.4 litre (2¼ pint) ring mould (see How to Line a Sandwich Tin,
Make the icing. Halve and scoop out the pulp from 2 of the passion fruit into a sieve resting over a small bowl. Press the pulp through the sieve, then measure out 4–5 teaspoons of the juice, pulping the third passion fruit if necessary. Stir in enough of the icing sugar to make a smooth, spoonable icing. Spread over the top of the cake so that it runs down the side.