Cook in boiling salted water for ten minutes, until partly cooked.
Make a sauce by melting the butter; into which stir the flour, a pinch of salt, the chicken stock and cream.
Then stir in the egg yolks and finally the grated Parmesan cheese. Stir until all the cheese melts.
Do this over a low heat.
In a buttered oven-proof dish put the fiddleheads carefully in layers. Pour sauce over each layer; sprinkle a little Parmesan on each layer of sauce.
Cover the top layer of fiddleheads with sauce and sprinkle with breadcrumbs.
Bake until sauce is bubbly.