Mix 1 cup butter, 1 cup brown sugar and 2 eggs. Stir in 2 cups flour and 1/2 teaspoon salt. Press in ungreased 9 x 13 pan.
Bake for 25 minutes. Let cool.
Heat chocolate and 1/2 cup butter over low heat, stirring constantly until melted. Remove from heat.
Stir in 2 cups powdered sugar and 2 eggs. Beat until smooth. Stir in marshmallows and pecans. Spread mixture over cookie-base. Refrigerate 2 hours. Cut into bars.