1Heat oven to 350° F (325° F for dark or nonstick pan). Grease or spray bottom of 13x9-inch pan.
2Make brownie batter as directed on box; stir in bananas. Spread into pan. Bake 24 to 26 minutes or until toothpick inserted 2 inches from side of pan comes out almost clean. Cool completely.
3In food processor, process strawberries until pureed. In medium bowl, beat powdered sugar, strawberries, butter and vanilla with electric mixer on low speed until smooth. Spread over cooled brownies. Garnish with strawberry slices, if desired.