Combine yolks, lemon zest, lemon juice, and sugar in a small saucepan. Whisk to combine.
Cook over low to medium heat, stirring constantly with a wooden spoon, making sure to stir sides and bottom of pan. Cook until mixture is thick enough to coat back of wooden spoon, about 5 minutes.
Remove saucepan from heat. Add butter, one piece at a time, stirring with the wooden spoon until consistency is thick and smooth.
Transfer mixture to a bowl. Lay a sheet of plastic wrap directly on the surface of the curd to avoid a skin from forming; wrap tightly.
Refrigerate until firm and chilled, at least 1 hour.
To Assemble the Napoleons
Bake the puff pastry according to the package instructions. Be sure to prick the sheet of pastry with a fork so that it doesn't puff up too much. They will bake for approximately 10 - 12 minutes.
Add sugar to the whipped cream and whisk until peaks begin to form.
Once pastry has been removed from oven and allowed to cool, cut into equal size rectangles.
Each napoleon will require 3 rectangles. Place the bottom one down, and spread with some of the lemon curd. Place another layer of pastry on top, then spread with the cream. Finally, put the last layer on top.
Mix the confectioner's sugar with a few drops of milk until you reach a glaze consistency. You want it to be easily spreadable but not so runny that it runs off the top of the pastry. Glaze each napoleon and allow to chill in the fridge until the glaze has hardened.
Using a heavy bottom pan and constantly stirring will help avoid the lemon curd curdling and creating scrambled eggs! Do not be tempted to raise the heat above a medium temperature. Slow and steady is the way to go!