Combine first 3 ingredients in a medium saucepan. Bring to a boil over medium-high heat, stirring constantly with a whisk. Reduce heat to medium; cook 3 minutes or until chocolate melts, stirring constantly. Reduce heat to medium-low; add marshmallow creme, stirring until creme melts. Bring mixture to a simmer over medium heat, and remove from heat.
Add sour cream and vanilla, stirring until well blended. Serve warm with cake, bananas, and strawberries.