Cook bacon in a large saucepan or Dutch oven over medium heat until crisp. Add mushrooms and cook until tender.
Drain clams and reserve liquid. Add enough water to reserved liquid to measure 1-1/2 cups. Stir in clam soup base.
Add reserved clam liquid, potatoes, onions and garlic powder to mushrooms and bacon mixture. Cover and cook 15 to 20 minutes, or until potatoes are tender.
Combine cream, corn starch and pepper. Add to chowder with drained clams. Heat until slightly thickened and bubbly, stirring occasionally.
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