Preparation

Unfreeze the puff pastry and roll out a sheet on a floured surface a bit wider than the baking pan (50×30 cm/20×12 in). Turn the baking pan upside down and place the puff pastry sheet on it, pierce in few places with a fork and bake for 10 minutes in an oven previously heated to 220°C (428°F).

For the cream, dissolve the flour in part of the milk, making sure not to leave any lumps. Stir in the yolks and mix it all together with a whisk.
Leave the rest of the milk to boil.

Pour the hot milk over the yolks and from the moment the mixture boils, cook for 5 minutes, constantly stirring (after 5 minutes you do not stop cooking). After the cream has been cooking for 5 minutes, start mixing the egg-whites with an electric mixer, and when they start frothing, add little by little sugar. Whisked egg-white has to be shiny and firm, but you need to be careful not to over-whisk it, because it will not connect to the cream nicely. For the entire time the egg-whites are being whisked, the cream is being cooked and stirred.

Transfer the egg-whites into a larger bowl, where you will connect it with the cream. In an even flow pour the cream into the center of the egg-whites, and mix it in from the center of the cream outward for the entire time with circular movements. That way you get a nice, aired texture of the cream.

Place one sheet of baked puff pastry into a baking pan and pour the hot cream over it. Cut the second sheet in squares (6×4 cm/2.6×1.6 in) and cover the cream with them. Sprinkle them abundantly with powder sugar.