In a bowl, beat the eggs and sugar using an electric mixer.
Mix the flour into the eggs, then blend in 1/2 cup of milk.
Add 3 cups of milk into a saucepan and bring to a slight simmer, but don't let it boil.
Slowly pour the hot milk into the egg mixture while stirring constantly with a whisk.
Transfer the custard into the same saucepan and place it back onto the burner and heat over medium-low heat. Make sure to constantly stir until the custard thickens. This can take up to 15 minutes, so be patient. Remove the custard from the heat and place plastic wrap right on top of the custard to prevent a crust from forming. Let it cool completely.
Thaw out each sheet of puff pastry and place onto a floured surface. Using a rolling pin, roll each sheet out to a 12 x 16 inch rectangle.
Using the rolling pin, transfer the puff pastry sheet onto a baking sheet. Then generously puncture the puff pastry with a fork.
Bake in a 400 Fahrenheit degree oven for 20 minutes or until golden brown.
Once the custard has cooled completely, add in the softened butter but only 2 Tbsp at a time with mixing in between. This will help prevent separation.
Lastly, add in the vanilla and thawed cool whip. Mix just to combine and the custard is done.
Divide the cream amongst 5 layers of puff baked pastry sheets and reserve the 6th sheet.
Crumble the reserved puff pastry sheet and coat the top and sides of the cake with it. At this point, you can either leave the cake as is or decorate it with fresh berries.
Video
Notes
Stir the hot milk into the eggs quickly- Stir quickly while slowly adding the hot milk because you don't want to scramble the eggs.
Add the butter a little at a time- When adding the butter to the custard, mix in a couple of tablespoons at a time to avoid separation.
Thin out the custard- If your Napoleon custard has turned out too thick, then thin it out by mixing in some more milk, or even folding in some whipped cream.