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1
Place the cubed Fontina in a bowl and pour the cold milk over it. Cover and refrigerate for 4–6 hours, or overnight. The cheese will soften and the milk will take on a pale gold tint. The soak is complete when the cubes yield perceptibly to the touch.
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2
Transfer the soaked cheese and all its milk to a heatproof bowl set over a pan of barely simmering water — the bowl must not touch the water. Stir continuously with a wooden spoon in a figure-eight pattern, keeping the bagnomaria temperature below 70 °C. The mixture will appear stringy at first, then gradually resolve into a smooth, homogeneous cream. This step is where the dish lives or dies: too much heat or too little stirring and the cheese breaks into an oily, grainy mass that cannot be recovered. The melt is complete when no solid pieces remain and the cream coats the spoon in an unbroken, glossy film.
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3
Remove the bowl from the heat. Whisk the flour into the egg yolks until smooth, then add them to the fonduta one at a time, stirring rapidly after each addition. The residual heat cooks the yolks gently into a velvety, custard-like body without scrambling them. The mixture should deepen in colour and thicken perceptibly. It is ready when it falls from the spoon in a slow, unbroken ribbon.
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4
Add the butter piece by piece, stirring each piece to full incorporation before adding the next. The fonduta will take on a final gloss and a supple, pourable consistency. It is ready when it moves in the bowl with the slow, fluid weight of a light custard sauce.
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5
Pour immediately over toasted bread, soft polenta, or dressed gnocchi. If white truffle is available, shave it directly over the fonduta at table: its perfume should rise freely from the heat of the dish. Serve without delay — fonduta stiffens as it cools.