Preheat oven to 350 degrees. Grease 9xpan13 inch .

Beat butter, shortening, granulated sugar and brown sugar in a large bowl until light and fluffy. Add eggs, one at a time, beating after each. Add vanilla and mix well.

Combine flour, baking soda and salt in small bowl. Add to creamed mixture and beat until thoroughly combined. Gently fold in marshmallows and spread into prepared pan.

Bake 15 to 20 minutes or until golden brown. Cool in pan before cutting into bars. Store in an airtight container.