Melt butter in a small Dutch oven over low heat. Add zucchini, shallot, and garlic; cover and cook 15 minutes, stirring occasionally.
Place mixture in container of an electric blender; add 1 cup chicken broth. Process until smooth. Pour mixture into Dutch oven; add remaining broth, whipping cream, salt, and curry powder. Cook over medium heat, stirring frequently, until hot.
Ladle soup into individual serving bowls; garnish with croutons, and serve warm. Soup may also be served chilled.