Preheat oven to 350 degrees.
Combine graham cracker crumbs, gingersnap crumbs, and butter in a medium bowl. Press into the bottom of a 9×13-inch baking pan.
Place in oven and bake for about 10 minutes, until golden brown. Place on a wire rack to cool.
In a medium saucepan, combine 2 ¾ cups milk, banana puree, ¾ cup sugar, and vanilla. Cook over medium heat, whisking frequently, until mixture boils. Once it boils, remove pan from heat and set aside.
Combine ¼ cup milk and gelatin in a small bowl and let sit for 5 minutes to soften the gelatin.
While gelatin is softening, whisk together egg yolks, remaining ¼cup sugar, and flour until smooth. Set aside.
Add gelatin to milk/banana mixture, whisking well to combine. Ladle about 1 cup hot milk mixture into yolk mixture, whisking constantly to combine. (I’m always paranoid about curdling the eggs, so I very gradually whisked in about 1 ½ cups of the hot mixture. The idea is to gradually raise the temperature of the eggs.) Pour egg mixture back into milk mixture, whisking continuously. Cook mixture over medium-high heat, stirring constantly, until mixture begins to thicken. Boil gently for 2 minutes.
Remove from heat and strain through a wire-mesh sieve. (I used a fine-mesh sieve and this part was quite a chore. Had to mush it through with the back of a spoon and the process was tedious.)
Spread half of mixture onto crust, scatter with vanilla wafers and then cover with remaining mixture. Let cool to room temperature and then refrigerate for at least 6 hours.
Combine cream and confectioners’ sugar in a medium bowl and beat at medium speed with an electric mixer until medium peaks form. Spread on top of cooled filling, Cut into squares to serve and garnish with mini vanilla wafers. Store in refrigerator for up to 5 days.