Heat oven to 350°F. Line 13x9-inch pan with foil, extending foil over 2 opposite sides of pan; spray with cooking spray. In 2-quart heavy saucepan, melt chocolates and butter over low heat, stirring constantly. Remove from heat; cool. In large bowl, beat sugar, eggs and vanilla with electric mixer on medium speed until thick and pale, about 5 minutes. On low speed, beat in flour, salt and melted chocolate until blended. Stir in marshmallows and peanuts. Pour into pan.