Place the lemon zest in a medium bowl and set aside. Spoon 4 tablespoons of the lemon juice into a very small bowl, sprinkle with the gelatin and set aside to thicken and almost set.
Place the eggs, egg yolks, sugar, salt, and remaining lemon juice in the bowl with the lemon zest and whisk together. Pour into a saucepan, add the butter and cook over med-low heat, whisking constantly, until the custard begins to thicken. Just before boiling remove from the heat and add the now thickened lemon gelatin mixture. Pour the whole mixture through a fine mesh strainer set over a medium bowl. Cover the surface directly with plastic wrap and chill overnight until set quite firm.
The next day, preheat the oven to 350 degrees F.
Unwrap the sheets of fillo and cover with damp cloth or a piece of plastic wrap. You should work quickly to avoid the fillo drying out. Lay two pieces of parchment paper measuring about 16 by 11-inches out on large chopping board. Brush each piece of paper with a thin layer of butter and sift two tablespoons of confectioners' sugar onto each one.
Lay a sheet of fillo onto each of the papers and now brush that with butter also. Continue to layer the fillo and butter on each stack to give 4 fillo layers in total, finishing with a butter layer. Finally, sift 2 tablespoons of confectioners' sugar on top of each. Lay another sheet of parchment paper measuring 16 by 11-inches on top and use this as a guide to trim the pastry edges using a sharp knife.
Next, remove the top sheet of parchment and using the knife, divide each fillo stack in half down the length (to give four 5.5-inches wide and 16-inches long rectangles). Make light score marks (not cutting through) with the knife across the opposite way and at 2-inch intervals to give 8 portion marks when serving. Slide each stack onto a sheet pan, butter the top sheets of parchment paper and place them butter side down on top. Finally, place another sheet pan on top of each stack (which will ensure the pastry will cook thin and crispy).
Bake for 15 minutes, until crisp and golden brown. Remove the top sheet pans and allow the pastries to cool completely.
To assemble, set one long rectangle onto a large platter and spread over 1/4 of the lemon cream evenly. Continue to layer up the fillo and lemon cream ending with a fillo top, to give 4 fillo layers and 3 lemon cream layers. There should be a remaining 1/4 of lemon cream still in the bowl to make a sauce with. To do this, add a 1/4 cup of hot tap water and whisk until smooth and drizzly in consistency.
Finally, prepare the blackberries. Tip them into a small bowl and lightly crush them with a fork to bring out their juices. Add 1 tablespoon confectioners' sugar and toss together.
Using a very sharp knife carefully cut the Napoleon into 8 portions using the score marks as a guide. Drizzle some lemon sauce on each serving plate and place a portion of Napoleon in the center of each one. Spoon the crushed blackberries around and serve at once.