Butter sides of 2qt sauce pan, and bottom and sides of either 9x13 pan or 11x7 pan. Measure out the marshmallow, PB, chips and vanilla so they are ready to add quickly.

Combine sugar and milk in 2 qt pan. Stir over medium heat til sugar dissolves and mixture boils. Cook to soft ball stage--234 degrees on candy thermometer...this takes quite a while.

Remove from heat and quickly add remaining ingredients. Blend until chips are melted. Pour into cake pan, spread even. Score top half and refrigerate to set. Once firm, cut all the way through before lifting pieces (otherwise it crumbles).