On medium-low heat, reduce the cream by one-third. In a separate pan, saute the green onions and mushrooms in butter. Season with salt, white pepper, and cayenne pepper. Cook the mixture for 10 minutes over high heat, stirring regularly. Add the white wine and reduced cream. Bring to a boil. Remove from the heat and allow to cool briefly. Whisk in the yolk and bring back to a boil, stirring constantly. Remove from the heat and refrigerate until final assembly.
