For the filling, put the cornstarch, sugar and egg in a pot. Beat the ingredients together and cook on a medium heat. When boiling, cook for two minutes, stirring to prevent lumps from forming.
Remove the mixture from the heat and add the olive oil. Cover the pot with transparent film and leave to cool.
Add the cream or light cream, mixing with a spatula until the mixture is uniform. Refrigerate.
To make the mixture, first sprinkle the flour over a clean, dry surface, then add the olive oil. Mix the ingredients together.
In a bowl, mix the cold water, the spirit vinegar and egg, and incorporate into the previous mixture. Knead lightly.
Divide the dough into 11 parts and make balls of the same size, covering them with transparent film for 30 minutes.
Flour each ball and flatten them, maintaining the circular shape. Roll each one out with a rolling pin, maintaining the circular shape.
Use an 8 inch cutter or plate and cut the layers with a knife.
Prick the circles of dough with a fork and refrigerate for half an hour.
Cook the 11 circles of pastry and the cutoffs for 15 to 18 minutes in an oven preheated to 390°F. Leave to cool.
To assemble the Napoleon cake, sandwich the layers of cake with the cream and cover the top and sides. Blend the baked cutoff pastry and sprinkle on top to decorate. Refrigerate for 30 minutes and serve cold.