Sauté halibut in 1 tablespoon butter over medium-high heat 3 to 5 minutes on each side. Let cool slightly; break fish into bite-size chunks. Set aside.
Sauté leeks in a Dutch oven in remaining 1 tablespoon butter 5 minutes or until leeks are soft. Add sweet potatoes and broth. Bring to a boil; cover, reduce heat, and simmer 25 to 30 minutes or until potatoes are tender.
Puree soup in a blender or food processor until smooth, and return to Dutch oven. Stir in basil, whipping cream, and halibut. Cook over medium heat 5 minutes or until hot. Season with salt and white pepper, and ladle soup into individual serving bowls. Garnish, if desired.