In 5-quart Dutch oven, heat butter over medium-high heat. Stir in celery and onion. Cook 10 minutes, stirring frequently, until vegetables are tender. Add potatoes, broth, bouillon, and pepper. Bring mixture to a boil; reduce heat to medium-low. Cover; simmer 30 minutes, stirring occasionally, until potatoes are tender. Let cool15 minutes before processing in food processor.