How to Make It
In a medium heatproof bowl, combine the grated cheese and the milk. Allow the mixture to stand for 20 minutes and then drain off the milk, reserving 1/4 cup. Set the bowl with the cheese over a saucepan of barely simmering water, or transfer the cheese to the top of a double boiler. Whisk until the cheese melts, about 5 minutes.
In a small bowl, whisk together the egg yolks and the reserved 1/4 cup milk and add to the melted cheese with the butter, whisking. The mixture will separate. Add the salt and pepper and continue whisking until the sauce is almost smooth, about 3 minutes longer. Do not cook too long or the mixture will curdle. The sauce will not be completely smooth.
Meanwhile, in a large pot of boiling, salted water, cook the fusilli for 10 minutes. Add the zucchini and bring back to a boil. Cook until the zucchini and pasta are just done, about 3 minutes longer. Drain. Toss with the fonduta.
Seeding Zucchini Because the zucchini are boiled here, we recommend seeding them first; otherwise, the pulpy seeds become water-logged. To remove the seeds, cut the zucchini into quarters lengthwise. Then simply cut off the seeds.