Melt butter in a small Dutch oven over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add chicken stock, milk, and whipping cream; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and pepper. Add diced chicken to Dutch oven; cook over medium heat, stirring constantly, 3 minutes or until thoroughly heated. Ladle soup into individual bowls; top each with chopped celery leaves. Serve immediately.