- Heat oven to 350º F.
- In a bowl, whisk 1 cup sugar with 6 eggs. Stir in 2 1/2 cups whole milk and 1 tablespoon vanilla.
- Pour into 4 custard cups or ramekins; set them in a baking pan. Pour hot water into the pan so it comes halfway up the sides of the cups. Bake until the custard is set, about 45 minutes.
- Remove the cups from the hot water, cool briefly, then refrigerate until set, about 1 hour.
- Peel and section 3 oranges. Toss the sections with 2 tablespoons sugar, 1 teaspoon vanilla, and 1 tablespoon rum, if desired. Spoon the marinated fruit over each custard and serve.