How to cook
For the preparation of the layers, first melt the butter and sift the flour. Mix all the products, beat with mixer and finally begin adding the flour until the dough tightens. Leave it covered in the refrigerator for 2 hours.
Then divide the dough into 12 equal parts, roll them out on a floured surface into thin sheets. Try to make them as thin as possible. Use a large plate as a template with which cut out the circles. Preheat the oven to 360°F (180 °C) and bake the sheets one by one in a slightly greased baking dish.
Baking needs to happen very fast, about 3-5 minutes per sheet. Leave the ready ones to cool a bit and in the meantime prepare the cream.
Put the milk (excluding 4/5 cup (200 ml)) on the stove with 2/3 cup (150 g) of sugar to boil. In a separate bowl, whisk the egg yolks with the remaining 2/3 cup (150 g) sugar. Once the milk comes to a boil, remove from the heat and start adding the flour a little at a time, while constantly beating. Then start adding small amounts of the milk mixture to the eggs and keep beating.
Put thus prepared cream back on the stove on low heat, and in the meantime beat the whites into foam with a pinch of salt. Once the cream has thickened, remove it from the heat and mix thoroughly with the egg whites. Return to the stove and boil on low heat until completely thickened. Whipping the cream is a long process that requires patience - it's important for this to happen on low heat.
Leave the finished cream for the Napoleon cake to cool slightly and add the soft butter to it. Begin to assemble the cake, alternating between cake layer and cream, until you run out of them. Top off with plenty of cream. Leave the cake to sit for at least 12 hours for the cream to tighten and for the cake layers to soften.