Begin by preheating your oven to 350°F (175°C). Lay each puff pastry sheet on a separate baking sheet and bake for 10 minutes, or until each sheet turns a lovely golden brown. Once baked, set them aside to cool.
In a saucepan, pour in three cups of the milk and gently bring it to a boil over medium heat. This will form the base of your custard.
While the milk heats up, whisk together the egg yolks and granulated sugar in a large mixing bowl until light and fluffy. Add the flour, vanilla extract, and the optional rum for an extra kick. Mix in the remaining 1 cup of milk to create a smooth blend.
Once your milk has reached a boiling point, carefully fold in your egg yolk mixture. Stir slowly and continuously until the custard begins to thicken. Once thickened, remove the saucepan from the heat.
In a separate large bowl, use an electric mixer to beat the egg whites until stiff peaks form. Gently fold these beaten egg whites into your custard mixture, ensuring it’s well combined without deflating the egg whites.
Take one of your cooled puff pastry sheets and place it at the bottom of a 9×13-inch baking dish. Pour your custard over the pastry, spreading it evenly. Chill in the refrigerator until the custard is cool and firm, about 1-2 hours.
After the custard has set, beat the heavy whipping cream with 2 tablespoons of powdered sugar until stiff peaks form. Spread this whipped cream evenly over the chilled custard layer.
Cut the second puff pastry sheet into 15 squares and delicately place these squares over the whipped cream layer. Chill in the refrigerator for another 2 hours.
Before serving, dust the Vanilla Custard Cream Squares with additional powdered sugar for a touch of elegance.