Melt butter in a large Dutch oven; add onion and next 3 ingredients and saute over medium heat 3 minutes. Add sweet potato cubes and chicken broth; cook over medium heat 30 minutes, stirring occasionally.
Process mixture, in batches, in an electric blender or food processor until smooth. Return mixture to Dutch oven; whisk in half-and-half and peanut butter. Reduce heat to low and simmer 15 minutes, stirring often.
Serve with Nutmeg-Molasses Cream and chopped toasted pecans.
For the Nutmeg-Molasses Cream, beat whipping cream at medium speed with an electric mixer until soft peaks form; gradually beat in molasses and nutmeg. Chill until ready to serve.