1. Preheat oven to 350°.

2. In a medium bowl, combine Graham-cracker crumbs, gingersnap crumbs, and butter. Press into the bottom of a 9x13-inch baking pan.

3. Bake until golden brown, approximately 10 minutes. Remove from oven, and let cool on a wire rack.

4. In a medium saucepan, combine 2¾ cups milk, banana purée, ¾ cup sugar, and vanilla. Cook over medium heat, whisking often, until mixture boils. Remove from heat. 

5. In a small bowl, combine remaining ¼ cup milk and gelatin. Let stand for 5 minutes. Meanwhile, in a medium bowl, lightly whisk egg yolks, remaining ¼ cup sugar, and flour until smooth. Set aside. 

6. Add gelatin mixture to milk mixture, whisking to combine. Ladle about 1 cup hot milk mixture into yolk mixture, whisking constantly to combine. Pour egg mixture back into milk mixture, whisking constantly to combine. Cook over medium-high heat, stirring constantly, until mixture begins to thicken. Gently boil for 2 minutes. Remove from heat, and strain through a wire-mesh sieve. Spread onto prepared crust. Let mixture cool to room temperature. Refrigerate for at least 6 hours. 

7. In a medium bowl, combine cream and confectioners’ sugar. Beat at medium-high speed with an electric mixer until medium peaks form. Spread onto cooled custard. Cut into bars. Garnish with a mini wafer, if desired. Store, covered, in refrigerator for up to 5 days.