Lay cheese slices in a 4-cup (32-fl oz/1-liter) bowl, overlapping them slightly. Pour the milk and cream over the cheese. Let stand for at least 2 hours, or overnight. When ready, drain the cream and milk and measure out 1/2 cup (4 fl oz/125 ml). In a small saucepan, heat the cream and milk over low heat until bubbles form around the edges of the pan. Stir in the butter.
Put cheese in a double boiler over simmering water and stir constantly until melted. Gradually stir in the cream and milk until smooth and creamy. Gradually beat in egg yolks. The mixture should thicken slightly. Immediately transfer to a warmed fondue pot. If using, scatter the truffle slices or drizzle the truffle oil on top.
At the table, skewer bread slices onto forks and dip into the fondue, or spoon the fondue over individual bowls of rice to eat with a table fork. Because of this dish’s egg content, it should be maintained over a very low flame and eaten immediately.