1Heat oven to 350ºF (325ºF for dark or nonstick pan). Grease bottom only of 13x9-inch pan with shortening or cooking spray. Make brownies as directed on box. Cool completely, about 1 hour.
2In chilled large bowl, beat whipping cream with electric mixer on high speed until stiff. Gently stir in marshmallows and cherries. Spread evenly over brownies. Drizzle chocolate over top. Swirl chocolate through whipped cream mixture with knife if desired.
3Cover and refrigerate about 1 hour or until chilled. For servings, cut into 4 rows by 4 rows. Cover and refrigerate any remaining dessert.