Bring a large saucepan of salted water to the boil over high heat, add the macaroni and boil for about 8 minutes or until the macaroni is tender. Drain and set aside.
Heat the milk in a saucepan with the bay leaf until just warm – you don’t want it to boil. Melt the butter in a separate saucepan over medium high heat until foamy, then add the flour and stir constantly for 1-2 minutes until the mixture leaves the side of the pan.
Remove the pan from the heat and add the warm milk a little at a time, whisking after each addition until smooth and combined. Return the pan to medium heat and stir until the mixture begins to boil. Add the nutmeg and season to taste. Add the macaroni, cheddar, gruyere and half the parmesan and stir until melted and combined. Remove and discard the bay leaf, then stir in the grated truffle.
Spoon the macaroni cheese into a 3 litre baking dish and scatter over the breadcrumbs and remaining parmesan. Place in the own and bake for 40 minutes or until golden brown. (Alternatively, spoon the mixture into smaller dishes for individual servings, scatter over the breadcrumbs and parmesan and bake for 25-30 minutes.)
Serve immediately with crusty bread and a green salad, if you like.