Bake the layers from store-bought puff pastry for 25 minutes at 200 °C.

In a saucepan, combine the egg, sugar, starch, and vanillin, mix, pour in the hot milk, and mix again.

Put the mixture over heat and cook, stirring, until thickened and bubbling.

Remove from heat, transfer to another bowl, immediately add the butter, and mix. Press plastic wrap directly onto the surface, let cool, and chill for a couple of hours.

Whip the chilled heavy cream and fold it into the custard base - the cream is ready.

Layer the pastry sheets with cream and sprinkle crumbs on top.

Let the cake sit for an hour at room temperature, then chill in the fridge for another couple of hours.