Heat a Dutch oven coated with cooking spray over medium-high heat. Add onion, celery, and carrot; cook 4 minutes, stirring frequently.
Add cauliflower, broth, curry powder, and pepper; bring to a boil over high heat. Cover, reduce heat, and simmer 20 minutes or until carrot is tender.
Remove from heat; stir in 1 cup milk. Place 1 cup cauliflower mixture in a blender; process 10 seconds or until smooth, and pour into a large bowl. Repeat procedure with remaining cauliflower mixture.
Return cauliflower mixture to pan; stir in remaining 1 cup milk and salt. Cook over medium heat until thoroughly heated, stirring frequently. Remove from heat; add butter, stirring until melted. Garnish with celery leaves, if desired.