Lightly grease a 9x9 or 8x8-inch square pan
Mix margarine, sugar and milk in a heavy 3-quart saucepan; bring to a full rolling boil on medium heat, stirring constantly.
Continue boiling 5 minutes on medium heat or until candy thermometer reaches 240°F stirring constantly to prevent scorching. Remove from heat.
Stir in the peanut butter.
Gradually stir in the peanut butter chips until melted.
Add the marshmallow cream and vanilla. Stir until smooth and blended.
Pour into prepared pan and cool at room temperature.
When cooled, cut into squares and store in an airtight container