Place the chopped sweet potato and 1 1/2 cups water (360 ml) in a blender. Blend until as smooth as possible, stopping to scrape down the sides if needed.
Pour the sweet potato mixture into a medium saucepan. Cook over medium heat, stirring often with a whisk or heat-safe spoon, until it reaches a gentle boil. Reduce the heat to low and simmer for 10 to 12 minutes, stirring frequently so the thickening porridge does not stick.
Stir in the coconut milk, condensed milk, brown sugar if using, cinnamon, nutmeg, and salt. Simmer gently for 5 to 7 minutes, stirring often, until the sweet potato is fully cooked and the porridge is creamy.
If you prefer a thicker texture, stir in the cornstarch slurry. Cook for 1 to 2 minutes, stirring constantly, until thickened. If the porridge becomes thicker than you like, loosen it with some or all of the remaining 1/2 cup water (120 ml).
Remove the saucepan from the heat and stir in the vanilla. Taste carefully and add a little more condensed milk or brown sugar if desired. Serve warm.