1. Combine the flan mixture (from the box) and milk in a medium pot over medium heat.
2. Stir the mixture constantly and let it cook for about 5 minutes 
3. Turn off the heat and let the mixture sit for 10 minutes. 
4. Transfer the mixture to the refrigerator and chill it for 2 hours, or until it sets completely.
5. Pour the curdled flan, condensed milk, and rum in a blender and blend until everything is mixed well.
6. Pour the mixture into a glass bottle, cover it, and keep it refrigerated.