In a bowl, cream butter and sugar. Add flour, egg yolks, orange juice, grated orange rind and dark rum.
In a separate bowl, place egg whites and sea salt. Beat egg whites until stiff. Fold the egg whites into the first bowl with the rest of the ingredients.
In a sauce pan on medium heat, add the ¼ cup of sugar and melt completely until an amber color. Immediately pour the caramel into 4 custard cups. Make sure to turn each cup to spread sauce evenly.
Pour the custard into each cup. Place the cups in a pan filled ¼ full of water. Bake for 30 minutes at 375°F or until custard sets.
When the center of the custard is no longer runny, remove from oven. This dessert can be served warm or cold. For best results, allow to set in refrigerator over night.