Prepare the béchamel: put the butter in a small (heavy-bottomed) pan over a low heat. As soon as it has melted, add the flour and stir for 2-3 minutes with a wooden spoon, without letting the mixture brown. In the meanwhile bring the milk to the boil. Pour the boiling milk into the pan, a little at a time, and amalgamate it with the flour and butter, stirring constantly.
Cook for roughly 10 minutes over a very low heat, continuing to stir so that the sauce doesn’t stick to the pan. Turn off the heat and season with salt and nutmeg. Leave the béchamel to cool. Press the ricotta through a sieve into a bowl; add the béchamel (now cold), the eggs, the flakes of truffle (setting aside a few to use as a garnish) and two pinches of salt. Mix with a whisk until everything is well amalgamated and smooth. Grease the moulds with butter and dust with flour, then fill them, up to 1 cm from the top, with the mixture. Put a centimetre or two of water in the bottom of a baking tin, place the flan moulds in it and cook in a preheated oven, at 180 °C, for 50 minutes. At the end of this time, open the oven door but wait for a while before removing the flans: otherwise they would “sink” too much due to the difference in temperature. They will sink anyway, this is normal, but they will remain soft inside. Let the flans cool for 15-20 minutes (if served hot they may fall apart) and before serving garnish them with the truffle flakes you set aside.