Place the sugar, butter and evaporated milk into a heavy saucepan.
Heat on medium high heat until boiling, stirring constantly. Let boil for 5 minutes, stirring constantly. You may need to turn the heat down if it starts to overcook.
Remove from heat and add marshmallows, peanut butter and vanilla. Stir until well blended.
Pour into a 9" square pan and cool. Cut into squares before serving.