Whisk in the flour and cook for 1 minute, stirring constantly. Whisk in milk, cream and vegetable stock. Add roasted cauliflower, curry powder and cayenne pepper, and stir to combine. Raise heat to bring to low boil, then turn heat to low, cover and simmer until cauliflower is soft and easily pierced with a fork, about 15 minutes longer. Add cheese and stir until cheese is fully melted into soup. Remove from heat.
Use an immersion blender to blend the soup into a smooth purée in the pot. (If you don't have an immersion blender, purée soup in a countertop blender, working in batches on low speed. Remove center plug from lid, cover lid with clean towel and hold down the lid while blending, or the hot soup will blow the lid off and make a mess in the kitchen.) Taste and adjust seasoning with salt and black pepper.
Ladle soup into bowls, sprinkle with the chives, if using, and more cheese, and serve immediately.