- Heat the carrot juice and cardamom in a medium saucepan over medium-high heat.
- Bring to a boil, reduce heat, and simmer for 15 minutes (the carrot juice will separate; this is okay).
- Mix in the cornstarch mixture and cook, stirring constantly, until the sauce thickens and coats the back of a spoon, about 5 minutes.
- Pour the sauce into a blender and process to incorporate the carrot juice.
- Use the sauce immediately, or cool quickly by setting in a bowl of ice and water.
- Store in an airtight container for up to one week in the refrigerator or freeze for about one month.