Preheat oven to 350°F (175°C).
Place each puff pastry sheet on a baking sheet and bake for 10 minutes, until golden brown. Set aside to cool.
Add three cups of milk to a saucepan and bring to a boil over medium heat.
Meanwhile, in a large mixing bowl, whisk together the egg yolks and granulated sugar until light and fluffy. Add in the flour, vanilla, rum (if using), and remaining 1 cup of the milk.
When the milk has come to a boil, fold the egg yolk mixture into the boiling milk and slowly stir until thickened. Remove from heat.
In a large bowl, beat the egg whites with an electric mixer until stiff peaks form, and slowly fold into the custard mixture.
Place one of the puff pastry sheets on the bottom of a 9×13-inch (23x33 cm) baking dish. Spread custard evenly over the top. Place in refrigerator to chill until cool and stiff, 1-2 hours.
When the custard has chilled, beat whipping cream and 2 tablespoons of powdered sugar with an electric mixer until stiff.
Spread whipped cream evenly on top of the custard.
Cut the second pastry sheet into 15 squares and place over the whipped cream. Return to refrigerator to chill for 2 more hours.
Sprinkle with powdered sugar before serving. Enjoy!