In a bowl , place the Fontina and cover with milk.
Soak for at least two hours to soften.
Melt the butter in water, pour in the mixture of Fontina and egg yolks and cook everything, stirring constantly, until cheese has melted completely.
Salt lightly, mix, and cook 5 more minutes, without allowing it to boil.
Pour into soup bowls, place truffle slices on top and serve accompanied by triangles of toast.