In a blender, process cashews, 2 cups milk, salt, and syrup for 1 minute. Transfer mixture to a saucepan, add cardamom, and bring to a boil, whisking. Reduce heat and simmer, covered, 15 minutes.
In a bowl, whisk remaining Tbsp milk and the arrowroot until smooth. Add to saucepan and thicken mixture over medium heat, whisking, 30 seconds. Remove from heat and stir in van illa.
Strain custard into a bowl and let cool slightly, then transfer to refrigerator. Serve chilled.