Preheat oven to 350 degrees F. Line a baking sheet with parchment paper. Line a 9x13 inch baking dish with parchment paper and spray with cooking spray.
Crush cornflakes into fine crumbs using a food processor. Melt butter in a medium bowl. Add sugar stirring to combine. Pour butter over crushed cereal, stirring until well combined. Spread a little more than half of the crumb mixture in the baking dish for the crust. Press down on the crumbs to form a crust. Bake the crust for 12 minutes or until deep golden brown.
Spread the remaining crumbs on a baking sheet and bake for 5 minutes or until lightly toasted. Let both cool completely.
Meanwhile, make vanilla cream filling. Combine milk, vanilla bean, and seeds in a medium pot. Bring to a simmer over low heat until warm. Discard vanilla bean.
Whisk egg yolks and sugar in a large bowl until creamy and well combined, about 1 minute. Slowly add warm milk to egg mixture, whisking constantly. Pour milk mixture back into the pot and bring to a simmer over low heat, whisking frequently. Continue to heat for 5 minutes or until slightly thickened.
Combine gelatin powder and water in a small bowl, stirring until gelatin has dissolved. Remove custard from heat and stir in gelatin, whisking until fully combined. Transfer custard to a heat safe bowl and let cool completely in the fridge for 2 hours or until set.
Whip heavy cream with powdered sugar in the bowl of a stand mixer until stiff peaks form. Remove custard from the fridge and whisk until smooth. Add ⅓ of the whipped cream and fold in until combined. Add another ⅓ of the whipped cream, folding in the mixture. Repeat with the remaining whipped cream.
Spread vanilla cream filling on cooled cornflake crust in an even layer. Sprinkle cornflake crumbs on top in an even layer, gently pressing down on the cereal to let it adhere to the filling. Chill in the fridge for at least 4 hours or overnight. Remove from fridge and slice into bars.
Notes
The cereal crust, crumb topping, and vanilla custard can be prepared ahead of time.