- Pre-heat oven to 180˚C/Fan 160˚C/Gas 4.
- Grease a 1.5 litre ovenproof dish.
- In a large mixing bowl, beat together the butter, sugar and lemon zest until pale.
- Add the egg yolks one at a time, then slowly beat in the lemon juice, passionfruit, then milk.
- Sift in the flour and mix the batter together, until smooth.
- In a separate bowl, beat the egg whites to form soft peaks. Then, fold into the pudding batter.
- Pour the mixture into the ovenproof dish. Place the dish inside a roasting tin. Transfer to oven, add boiling water to the roasting tin so that it comes halfway up the outside of the dish.
- Bake for 40-45 minutes until risen slightly and pale golden-brown.
- To serve, dust with icing sugar.