2. Melt the butter in a saucepan over low heat (similar to starting a roux) without browning.
3. Then add the flour all at once, off the heat. Whisk to avoid lumps.
4. Return to low heat and cook until the mixture is frothy.
5. Gradually pour in the broth while whisking.
6. Add the mushrooms, salt, pepper (taking into account the seasoning of the broth).
Let simmer over low heat for 30 minutes, allowing the sauce to reduce slightly.
7. Just before serving, add the egg yolks and cream. Mix well and heat slightly to thicken the sauce.