In a cup, whisk an egg with 1 spoon cognac, add water until you have 1 cup full, mix well.
Cold butter cut into small cubes, add flour, mix well, but don't worry about getting rid of the lumps.
Add egg mixture and mix well. Form 5 balls (approximately 150 g each), dredge in flour and wrap in plastic wrap. Let stand in the fridge for 2-3 hrs, or overnight.
Preheat oven to 430 F. Roll each dough ball into a very thin circle or square shape and bake 8-10 min until golden brown. Let it cool on the rack. Pick the least attractive one and crumble.
Filling:
Version 1, butter cream:
300 g butter
3/4 cup condensed milk (sweetened)
1 tbl spoon cognac
Soften the butter. Whip until light (if using mixer, run on low speed). Slowly add condensed milk and cognac, continue whipping until light and homogenous.
Version 2, custard:
Bring butter to room temperature. Boil the milk, reduce heat to low.
Eggs, sugar and flour whisk together until foamy. Slowly add egg mixture to the hot milk, mixing constantly so there are no lumps. Gently boil for a few minutes until becomes thick.
Remove from heat, let it cool.
Whip softened butter and add a few spoons of egg-and-milk mixture, mix well and continue adding the mixture.
Whip the cream for a few minutes. Let cool down to room temperature.
To assemble, put the biggest layer on the serving dish, spread with cream, and add another layer, pressing gently. Repeat with remaining layers. Spread cream on top and sprinkle with reserved crumbs. Optionally, sprinkle with some sugar and crumbled nuts.