Pre-heat the oven to 180°C/350°F. Grease a 2 quart oven safe dish (ceramic, glass,Pyrex custard cups) or 8 4 “ounce ramekins generously with cooking spray and set aside.
In a medium bowl, whisk together granulated sugar, salt and flour. Set aside.
In a Large bowl, whisk together the egg yolks, butter, milk, rum, lime zest, lime juice and passion fruit puree and pulp until fully combined.
Add the wet mixture to the flour mixture and whisk to just combine.
Using an electric mixer, beat the egg whites for about a minute then add about 1 tablespoon sugar , if desired - it 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 whites 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 half-way up the sides of ramekins, or casserole dish. Bake for 25 to 30 minutes or until the top of the cake or cakes turn light golden and puff slightly (do a test with your finger , they should feel spongy and spring back a bit but hold a shallow indentation,.
Let the cake cool to room temperature . Refrigerate for about 2 or more hours before serving or up to 24 hours.