In a 6-quart stockpot or Dutch oven, heat oil, butter and pepper flakes over medium heat until butter is melted. Add carrots and onion; cook, uncovered, over medium heat, stirring frequently, until vegetables are softened, 8-10 minutes. Add garlic and dried basil; cook and stir 1 minute longer. Stir in tomatoes, chicken stock, tomato paste, sugar, salt and pepper; mix well. Bring to a boil. Reduce heat; simmer, uncovered, to let flavors blend, 20-25 minutes. , Remove pan from heat. Using a blender, puree soup in batches until smooth. If desired, slowly stir in heavy cream, stirring continuously to incorporate; return to stove to heat through. Top servings with fresh basil if desired.