1Heat oven to 350°F. Spray 13x9-inch pan with cooking spray.
2In large bowl, beat 1 cup butter with electric mixer on medium speed until smooth and creamy. Beat in granulated and brown sugars, 1 teaspoon vanilla and eggs until smooth. On low speed, beat in flour, 3/4 cup cocoa, the salt and baking soda until soft dough forms. Stir in chocolate chips, marshmallows and cashews. Spread mixture in pan.
3Bake 15 to 20 minutes or until set. Cool completely, about 1 hour.
4In small bowl, mix all frosting ingredients except cocoa, adding enough of the half-and-half until frosting is smooth and spreadable. Frost brownies. Sprinkle with 1/4 teaspoon cocoa and additional cashews if desired. Let stand 30 minutes or until frosting is set. Cut into 6 rows by 4 rows.