Line an 8-inch square baking pan with foil, leaving a 2-inch overhang; mist with cooking spray.
Combine butter, sugar, cream and corn syrup in a large, heavy saucepan.
Place over medium heat and cook, stirring, just until sugar dissolves. Clip a candy thermometer to side of pan, taking care not to let it touch the bottom. Boil mixture without stirring until thermometer reads 234F, about 5 minutes.
Remove pan from heat, remove thermometer and let stand 10 minutes (do not stir).
With a clean heatproof spatula, stir in peanut butter, marshmallow crme and vanilla until no streaks remain. Scrape mixture into baking pan. Allow to cool completely on a wire rack, then refrigerate at least 1 hour. Using foil overhang, remove fudge from pan.
