For custard, whisk together 1½ cups granulated sugar, flour, and ¾ tsp. salt in a large saucepan. Gradually whisk in milk. Whisk in egg yolks. Bring to a boil, stirring constantly. Reduce heat to medium-low heat. Cook and stir 3 minutes. Remove from heat. Stir in 2 tablespoons butter and vanilla until butter is melted. Transfer to a large bowl. Place plastic wrap on surface to cover; cool completely at room temperature.

For caramel sauce, heat and stir remaining 1½ cups granulated sugar in a medium saucepan over medium heat 4 to 5 minutes or until sugar turns a deep amber. Remove from heat and carefully stir in remaining ½ cup chopped butter until melted. Gradually stir in ¾ cup heavy cream until combined; stir in remaining ¼ teaspoon salt. Return to heat and simmer 1 minute. Remove from heat and cool to room temperature.

Place cookies in food processor. Cover and process until finely ground; set aside.

Beat remaining 2 cups heavy cream and powdered sugar in large bowl with an electric mixer on high until soft peaks form (tips curl).

To assemble, reserve 2 tablespoon cookie crumbs and 2 tablespoon caramel sauce for garnish. Place 1½ cups cookie crumbs in bottom of a 3½-quart trifle dish. Layer with half each custard, bananas, caramel sauce and whipped cream. Repeat layers with remaining cookie crumbs, custard, bananas, caramel sauce and whipped cream. Chill 1 hour. Drizzle with reserved caramel sauce and sprinkle with reserved cookie crumbs before serving.