Whisk together first 5 ingredients in a large saucepan over low heat; cook, whisking constantly, 8 to 10 minutes or until thickened. Remove from heat; stir in butter and vanilla.
Layer 5 graham crackers, half of bananas, and half of pudding in a 13- x 9-inch baking dish. Repeat layers, using remaining ingredients. Cover pudding, and chill 6 hours.
Beat whipping cream and 2 tablespoons sugar at medium speed with an electric mixer until soft peaks form. Spread over pudding. Chill until ready to serve. Prep: 5 min., Cook: 10 min., Chill: 6 hrs.