Grease bottom and sides of 8" square pan with butter.
In heavy saucepan, cook first 3 ingredients, stirring constantly, until butter is melted. Stir in marshmallow cream. Heat to boiling over medium heat and boil about 5 minutes, stirring occasionally, to 234° on candy thermometer or until small amount of mixture dropped into cup of very cold water forms a soft ball that flattens when removed from water.
Remove from heat. Add butterscotch chips, stirring until smooth. Stir in pecans.
Spread in pan.
Let stand 2 hours or until firm.