Pour in 2 cups broth and peanut butter, and stir until smooth. Add remaining broth, tomato paste and pepper flakes, and bring to a boil. Reduce heat and simmer, uncovered, for 20 minutes, until sweet potatoes are soft, but not mushy. Season with additional salt. (Amount will depend on how salty your peanut butter is—add enough so that the flavors “pop,” but it doesn’t taste overtly salty.)