Grate the cold butter into a large bowl and add the flour. Using your fingers, smush the flour into the butter (or use a pastry cutter) until coarse crumbs form.
Juice the lemon into the same cup that the flour was measured in, add the salt, then top with water to fill the cup. There is no exact measurement for the water.
Pour the water with lemon juice and salt into the flour-butter mix and bring together until a dough forms. Lightly knead as necessary to form the dough into a log. Wrap tightly in plastic wrap and chill for an hour up to overnight.
The custard:
Combine 2 cups of the milk with the sugar in a saucepan and place on the stove over medium heat until it barely begins to boil.
In a separate bowl, add the remaining milk (or milk and cream) with the eggs and flour, whisk to combine.
Slowly stream the near boiling sugar milk into the milk-eggs-flour bowl, whisking constantly, until the milk-eggs-flour mixture warms. Pour the contents of that bowl through a fine mesh strainer back into the saucepan and place on the stove.
Whisk the custard mix constantly until it thickens like a pudding then remove from the heat, add the vanilla and butter, stir to combine. Cover with plastic wrap directly on the surface and place in the refrigerator to chill.
To bake the pastry dough:
Preheat the oven to 400° F (200° C). Or, using a convection oven, preheat to 375° F (190° C). Place a rack in the lower third of the oven.
Remove the dough from the refrigerator and discard the plastic wrap. Cut the log of chilled dough into 12 evenly sized slices.
Roll out the dough on either parchment or a silicone mat with a bit of flour and place both the dough and the surface material on a cookie sheet. Be sure to roll the dough much larger than the intended finished size and roll it paper thin, that it's see-through thin. It will shrink in size.
Dock the dough with a fork to prevent puffing and bake at 400° for 15-20 minutes or 375° convection for 15-16 minutes, until evenly golden brown and crispy then remove from the oven.
Using the lid of a pan or an inverted bowl and a knife or pizza cutter, immediately cut the baked pastry to the desired size such as 9 or 10 inches. Gather the cutoff bits into a bowl and once every layer has been baked, crush the bits into smaller pieces.
While one is baking, roll out another piece or pieces of dough so they’re ready to go into the oven right when one comes out.
To assemble:
Set aside about 1 1/2 cups of the custard in a small bowl, cover, and place back in the refrigerator.
Place one baked pastry round on the serving plate of choice.
Divide the remaining custard by the number of layers you have which is about 4 heaping or 5 tablespoons of the custard per layer, give or take. Spread a portion of custard evenly over the first layer of baked pastry. Stack another piece of baked pastry atop and repeat until all the layers are piled up.
Cover the cake with plastic wrap, give it a very gentle, overall even smush but don't break everything. Let it rest on the counter for 30 minutes then refrigerate for at least 6 hours but ideally overnight.
When ready to serve, remove the serving plate from the refrigerator, remove the plastic and coat the top and sides with the remaining custard. Coat the top and sides with the crushed pastry bits and serve or chill further then serve.
Notes
Adapted from Kitchen Russian (website no longer working).