Heat oven to 180C/160C fan/gas 4. Put a baking tray on the top shelf to heat up. Grease 4 x 150ml ramekins with butter and dust the insides with caster sugar to coat.
Whisk the egg whites in a bowl until stiff. Add 1 tbsp of the sugar and whisk for 30 secs more until thick and glossy. In a separate bowl, scoop the pulp from 1 passion fruit into the custard, then fold in the whisked egg white. Spoon the mixture into the prepared ramekins and put on the hot baking tray. Bake for 15 mins until risen and golden on top.
While the soufflés are cooking, make the sauce. Scoop the remaining passion fruit pulp into a saucepan, add the remaining sugar and simmer gently for 5 mins. Finally add a knob of butter and stir until melted. Pass through a sieve and serve in a jug with the hot soufflés as soon as they come out of the oven. Let your guests break a hole in the centre of their soufflé, then pour in the sauce.