Preheat the oven to 180°C/350°F. Grease a 2-quart oven-safe dish (ceramic, glass, Pyrex custard cups) or eight 4-ounce ramekins generously with cooking spray and set aside.
Whisk together granulated sugar, salt, and flour in a medium bowl. Set aside.
In a large bowl, whisk together the egg yolks, butter, milk, rum, zest, lime juice, passion fruit puree, and passion fruit pulp until fully combined.
Add the wet mixture to the flour mixture and whisk to combine.
Using an electric mixer, beat the egg whites for about a minute, then add about a tablespoon of sugar if desired - it's optional. Continue mixing on medium-high until stiff peaks form; this may take about 2-3 more minutes.
Using a spatula, carefully fold in ¼ of the egg white mixture, and then fold in the remaining until well incorporated.
Pour the batter into a buttered baking dish, place in the large baking or roasting pan, then carefully pour hot water into the baking pan to come halfway up the sides of the ramekins or casserole dish. Bake for 25-30 minutes or until the top turns light golden and puff slightly (do a test with your finger, it should feel spongy and spring back a bit but hold a shallow indentation.
Let the cake cool to room temperature. Refrigerate for 2 hours or more before serving (up to 24 hours).