PREHEAT oven to 350°F. Place 3/4 cup of the butter and 3/4 cup of the sugar in small bowl of electric mixer. Beat on medium speed until light and fluffy. Add flour; mix well. Press firmly onto bottom of 13x9-inch baking pan. Bake 12 to 15 minutes or until golden brown; cool.
PLACE chocolate and remaining 1/2 cup butter in large saucepan; cook on low heat until chocolate is completely melted, stirring constantly. Remove from heat. Add egg product and remaining 2 cups powdered sugar; stir until well blended. Add marshmallows and pecans; mix well.
SPREAD over crust. Refrigerate several hours or until chilled. Cut into 36 bars to serve. Store in tightly covered container in refrigerator.