Combine egg yolks & sugar in a bowl and whisk them together, adding 1/4 cup of milk to make it easier.
Add the flour and whisk again, forming a very uniform, clump-free batter. Add another 1/4 cup of milk to make it easier.
Add butter, allow to melt and stir to form a smooth custard. Alternatively, you can beat the butter into the cooled to room temperature custard.
Use 2 boxes of storebought frozen Pepperidge Farm puff pastry in place of homemade puff pastry if desired.
For this, roll out each thawed puff pastry sheet on a floured surface to a 20"x14" rectangle and poke it with a fork every 1 inch, then bake for about 8 minutes at 400F or until uniformly golden in color. Remove from the oven and allow to cool.
Trim the edges of the cake with a sharp knife to make all layers the same size.
Now cut 4 baked cake layers into two equal halves, giving you 8 cake layers.
Layer the 8 baked puff pastry cake layers with about 1 cup of prepared custard, then use the rest to cover the top and sides of the Napoleon cake.